Browsing by Author "Rijkenberg, Fredericus Hermanus Johannes."

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African fungus-growing termites and other insects for human and poultry nutrition.
(2004) Moore, Alexander Jackson.; Rijkenberg, Fredericus Hermanus Johannes.; Miller, Raymond Martin.
Food insecurity can contribute to the advancement of diseases such as growth stunting and HIV/AIDS. A holistic approach to addressing food insecurity includes reviewing local resources; including indigenous food stuffs. Six studies investigate the potential of insect nutrition to meet dietary needs in rural South Africa. A novel trapping method for Trinervitermes sp. is examined by parameters of time, sustainability and bait used. Local grass (Themeda triandra Forssk.) seemed to be the most effective bait, being significantly more attractive than loose mound soil (p=0.01), wet maize stalks (p=0.01) or cardboard (p=0.05). The trapping device was demonstrated as an effective tool in assessing the feeding preferences of Trinervitermes sp., which compete directly with cattle for grazing food resources. The chemical composition of Macrotermes natalensis alates (winged, wingless and fried), soldiers, and Odontotermes sp. alates (wingless) was determined. Alates were rich in fat, ranging between 49.2-60.6% (dry matter basis). The protein content ofM natalensis and Odontotermes sp. alates compared favourably to pork and chicken. Alates were high in glutamic, aspartic and alanine amino acids and low in methionine, serine and threonine. Amino acid digestion for broiler chickens was high, ranging between 87.6-96.1%. In an era where rural and urban cultures are rapidly merging, entomophagy may be discarded as an embarrassment or nonsensical practice. The high nutritional content of M natalensis and Odontotermes sp. should be publicised both to increase the awareness of their high quality as a food source for both poultry and human consumption and to avoid the abandonment of cultural practices that make sense.
Black wattle (Acacia mearnsii) and the fever tree (Acacia xanthophloea) in alleycropping systems.
(2006) Nhamucho, Luis Jeremias.; Rijkenberg, Fredericus Hermanus Johannes.; Everson, Theresa Mary.; Underwood, Timothy Michael.; Matakala, Patrick.
Alleycropping is an agroforestry technology of planting crops between rows of trees, preferable legumes to promote an interaction among them with positive benefits in terms of improving soil fertility and hence good crop yields. The technology has been tested with a variety of trees/shrubs species in association with crops (alleycropping) or with grasses (alleygrazing), sometimes with encouraging results and sometimes not, in a wide range of environmental conditions around the world. Research in alleycropping started in late 1970s and sinc~ then many publications have been released. However, little or nothing has been reported about this technology using black wattle and the fever tree, two nitrogen-fixing trees common in South Africa and reported as fast-growing species which produce a considerable amount of biomass within a short period of time. Due to that fact, a two-year trial was established in 2003 at the Ukulinga research farm, Pietermaritzburg, South Africa to evaluate the potential of the two tree species under alleycropping with maize and cowpeas as joint intercrops, under alleycropping with pumpkin, and under alleygrazing with Eragrostis curvula and with Panicum maximum. The trial assessed the crop yields and the biomass production from all the components, and their fodder digestibility using Neutral Detergent Fibre (NDF) and Acid Detergent Fibre (ADF) determinations. Additionally the changes in tree grovlth vaa."'i.ables (difu~eter, total height, total \lollhl1e and biomass) were mortitored to produce regression equations to predict those variables, one from another, using regression analysis. The diameter was taken at ground level (dgl) and at the height of 1.3 m, normally called diameter at breast height (dbh). The results showed that tree growth and biomass production were better in black wattle alleycropping than in association with the fever tree. The average dgl of black wattle after 12 months was 48mm and the average dbh was 36mlll. Over the same period the total tree height was about 406cm. A tree pruning was done to one-year old black wattle in the whole trial and the prunings produced about 5.6t/ha of fresh foliage biomass in the association with maize and cowpea and 4.5t/ha in the association with pumpkin. In alleygrazing the growth variables were similar to those obtained in alleycropping but the biomass production was considerably different. The prunings produced about 7.66t/ha of fresh foliage biomass. The dry matter biomass from the prunings was 1.96t/ha, 1.58t/ha and 2.68t/ha in the association with maize and cowpeas, pumpkin and E. curvula respectively. The dry matter was obtained from 4days- oven-dried samples and it was 35% of the fresh foliage biomass and 60% of the fresh woody biomass. The fever tree did not grow significantly during the study period and due to that fact, the species was discarded from the study. Similarly, because after several endeavours using different seed lots, P. maximum had germinated very unevenly, and this grass was excluded from the experimentation. Values ofNDF and ADF less than 35% are considered good, between 35% and 60% fair and poor if greater than 50%. Using is classification the NDF and ADF values from this study were good in pumpkin (30.5% and 29.9%) and cowpeas (36.5% and 46.9%) biomass, fair in E. curvula (41.9% and 39.9%) and maize stover (53.6% and 42.1%) and poor in black wattle (76.58% and 68.1%) foliage. If black wattle is to be used as fodder, it must be mixed with highly digestible fodder like P. maximum, Digitaria sp., and other legume plants, to increase animal intake and to avoid any risk of it becoming an animal hazard due to tannin effects. The regression equations produced linear relationships between dgl and age, and biomass and dbh. The other interactions were not linear. The best equations were obtained in the interaction between dgl and age (dgl= 4.8*Age -7.03; R2 =0.86; SE= 6.6), dgl and height (h= -0.03dg12 + 10.5dgl - 21.25; R2= 0.96, SE= 42.9, h= height), biomass and dbh (lny = 2.409*lndbh; R2=0.99, SE=O.O, Y= tree foliage biomass). During the study, monkeys, cattle, birds and bushbucks posed a threat to the success of the study due to damage they caused to the crops. It was possible to keep the damage below the critical levels, although at high cost.
The effects of outsourcing on rural communities and business performance in the selected areas of the South African forestry industry.
(2001) Ngcobo, Sakhile Glen.; Rijkenberg, Fredericus Hermanus Johannes.
The objectives of the study were to examine the socio-economic impact of outsourcing on forest-dependent communities, to evaluate the current outsourcing management system in Mondi Forests, and to assess the financial impact (costs savings) of outsourcing on Mondi Forests: Natal Region in KwaZulu-Natal. Participatory Rural Appraisal exercises were used to assess the socio-economic impact of outsourcing on forest-dependent communities. A questionnaire was then used on a sample of Mondi Forests personnel to evaluate the current outsourcing management system in Mondi Forests. Finally, cash-flow trends were used to assess the financial impact (costs savings) of outsourcing on Mondi Forests, Natal Region. Activities that have been outsourced include transport, harvesting operations, silviculture operations, security services, mechanical services, training, payroll services and information systems. Results show that outsourcing has impacted negatively on the livelihoods of some of the forest-dependent communities sampled. The negative impacts are mainly associated with a reduction in hygiene services offered to these communities and an increase in unemployment in some of these villages. Forest-dependent communities perceive outsourcing as the cause of poor relationship between the rural communities, forest companies and contractors as well as being the cause of high unemployment rates. Mondi Forests' personnel perceive that outsourcing goes beyond costs savings to include redirecting scarce resources toward core business activities. Lower morale and insecurity among forest staff members and negative views of the forestry companies by rural communities, unions and government departments were perceived as the disadvantages of outsourcing. Uncertainty with regards to employment continuity and social problems (e.g. increased forest fires, poor relationship with local communities) were perceived as outsourcing risks. Results show that Mondi Forests has implemented a number of strategies in order to reduce some of these risks and disadvantages by giving preference to their ex-employees when offering contracts, by providing start-up capital to emerging black contractors, and by initiating community development programmes in order to uplift the standard of living of the forest-dependent communities. Mondi Forests' personnel suggested that forest management. accounting, conservation management, and human resources are the best candidates for future outsourcing, since most other forest organizations in the world are already outsourcing them successfully. Most of these activities are still kept in-house in Mondi Forests. Results also show that outsourcing can be used with other rationalization methods to save costs. This is because costs savings in Mondi Forests: Natal Region, since 1998 to 2001 were .not only due to outsourcing, but also to some other management decisions such as reduced fertilizer and pesticides use. The total costs savings were R14.5 million from 1998 to 2001. Results support the need to strengthen the current rural development programmes in Mondi Forests, and to the establish Community Liaison Forums to improve communication with rural communities and to start community development programmes. A contractor upliftment programme was also recommended for developing the emerging black contractors. Lastly, company/contractor agreements must be reviewed in order to ensure that social responsibility aspects are appropriately managed. Outsourcing relationships need to consider the economic, environmental and social effects in order to be sustainable.
An epidemiological analysis of the Phytophthora and Alternaria blight pathosystem in the Natal Midlands.
(1980) Putter, Christoffel Antonie Johannes.; Martin, Michael Menne.; Rijkenberg, Fredericus Hermanus Johannes.
The history of the development in Natal of a forecasting service to warn of outbreaks of late blight disease caused by Phytophthora infestans is presented. The late blight pathogen and Alternaria solani, the causal organism of early blight disease, interact on potatoes and tomatoes to form a blight disease complex. Evidence is presented to show that it is expedient to manage this blight complex as a whole rather than to direct control at only one of the components in ignorance of the consequential enhancement of the potential of the other. In a search for an improved blight complex management strategy, factors concerning the possible existence of an annual migration of Phytophthora infestans inoculum, first postulated in the 1960's, along an east-west route across Natal, are collected and collated. Corroboration of the existence of the Phytophthora-pathway is given, inasmuch as it represents a serial outbreak of late blight along a temporal gradient. The possibility that the pathway is a manifestation of disease resulting from the erruption of pre-existing inoculum along an environmental gradient, can not specifically be excluded. However, the peculiar pattern of anabatic and katabatic winds along a river-valley network, superimposed on a continuous cropping pattern and its concomitant opportunity for blight to be endemic in the province, supports the postulated Phytophthora-inoculum pathway A fungicide spray trial was conducted in order to investigate the possibility of us i ng the pathway phenomenon as the framework for an improved blight control strategy and to explore the nature and level of the competitive interaction between Phytophthora infestans and Alternaria solani. This trial revealed that the interaction between the components of the blight complex was differentially altered by weather patterns and fungicide combinations. Treatments in which metalaxyl (Ridomil) alone was used for the control of late blight, gave a yield similar to those with propineb (Antracol), which inhibits A. solani primarily but also hus some negative effect on P. infestans. The yields from both these treatments were siguificant ly (p < 0,05) better than the yields recorded in the unsprayed control plots. A treatment in which Ridomil and Antracol were combined such that each was applied according to its recommended concentration, gave yield increases of 32,3% over the unsprayed control, although the yield from the Ridomil/Antracol treatment was not significantly greater (p < 0,05) than the yields recorded where either Ridomil or Antracol were used. A computer simulator, named GAUSE, was developed to simUlate the consequences of the competition between various combinations of P. infestans and A. solani. Results simulated by GAUSE corroborated those obtained from the field trial and support the conclusion that diseases of complex etiology require more than simplistic, univariate analysis of single cause-and-effect pathways. The competition quotient CQ is developed as a new parameter of competitive interactions. It is calculated as the ratio of the amount of disease in the absence of competition, to the amount of disease when the causal pathogen is competing with another pathogen in the same niche. The CQ may be calculated from various standard epidemiology statistics and it is used to demonstrate that the competitive displacement phenomenon places constraints on the interpretation and application of Vanderplank's basic epidemiology equations. A new pathosystems management concept namely the pathotope (pathos = suffering; topos = place0 concept, is introduced, having developed from the notion that epidemics have spatial as well as temporal attributes. Accordingly, an area in which individual farms are at the same level of probability at risk to disease, delimits the pathotope. The concept can be described at many integration lsvels and is presented as an important quantitative unit of comparative epidemiology. The pathotope concept accomodates such notions as are contained in the postulated Phytopnthora-pathway and is especially suited to integration with disease forecasting methods. An example of the application of the pathotope approach is presented and a strategy is proposed by which fungicide spraying is initiated and applied synchronously as determined by the degree of communal risk to attack and epidemic increase of disease. Within a pathotope, several common factors collectively determine the vulnerability of the group to disease. If a coherent, uniform strategy is to be developed and implemented by pathotope members, it is necessary that all members have access to the relevant information and that it be collected and disseminated conveniently and rapidly. A computer-based disease monitoring and mapping system which achieves these objectives is presented.
The epidemiology and control of Leptosphaeria maculans cause of Crucifer Blackleg, in KwaZulu-Natal.
(1996) Laing, Mark Delmege.; Putter, Christoffel Antonie Johannes.; Rijkenberg, Fredericus Hermanus Johannes.
The perfect stage of Leptosphaeria maculans is reported for the first time in South Africa. Viable pseudothecia and pycnidia were found on dead, weathered tissue, sometimes in close association, whereas only pycnidia were found on live tissue. Some seedlots of imported cabbage seed were found to be internally infected with L. maculans at low levels and Alternaria brassicicola at higher levels. Fungicides iprodione (dicarboximide), triforine and propiconazole (sterol-biosynthesis inhibitor) eliminated both pathogens from infected seed. In a field trial of eight cabbage and two cauliflower cultivars, incidence of stem infection by L. maculans ranged from 16-80%. Two seedlots of the cabbage cultivar Gloria Osena differed in blackleg stem susceptibility. No correlation was found between stem lesion incidence and foliar infection counts of each cultivar, or stem lesion incidence and each cultivar's average days-to-harvest. In a second trial, incidence of stem infection ranged from 50% (Rotan) to 95% (Dynasty) in cabbage, and 64.2 to 96.6% in cauliflower cultivars. All Brussels sprouts and broccoli cultivars tested were highly susceptible. The cultivars of turnip and tyfon tested were observed to be immune to blackleg, whereas the swedes, Japanese radish, chou moullier and red cabbage cultivars tested were highly susceptible. No correlation was found between stem length and incidence of stem infection. Different seedlots within several cabbage and cauliflower cultivars differed in their blackleg susceptibility. A third cultivar trial with 10 replicates of four seedlots of one cabbage cultivar confirmed that different seedlots of a single cultivar may vary significantly in their susceptibility to blackleg. Benomyl was applied to cabbage at the seedling stage only, or at the seedling stage followed by field applications every 14 d. Relative to an untreated control, multiple applications of benomyl resulted in a 33% reduction in stem infection, a ten-fold reduction in plants killed and a 50% reduction in the proportion of non-harvestable heads, relative to an untreated control. Seedling treatment resulted in a lower infection level, a lower mortality rate and a greater mean head mass than those of the untreated control. However, none of these differences were statistically significant. In a debris degradation trial, more than 90% of buried debris (cabbage stems infected by L. maculans) had decomposed after 2.5 yr, whereas 80% of surface debris had decomposed over the same period. The susceptibilities of seedbed transplants (SBT) and container-grown seedlings (CGS) were compared using different forms of L. maculans inoculum. "Dunk" inoculation of SBT into a pycnidiosporial suspension resulted in a stem infection level of 50% greater than an uninocu1ated control. Contamination of seedbeds resulted in an infection level of 46%. "Dunk" inoculation of CGS resulted in infection level of 22%. When CGS were grown in contaminated trays an infection level of 33.4% resulted. Interplot interference ill the form of inoculum dispersal over a 1 m border was low (1.8 and 2.7% for SBT and CGS, respectively) . In a further trial examining the relationship of inoculum level and blackleg, a strong interaction was found between inoculation technique and inoculum level. Inoculation of field plots with infected debris was a more efficient technique than dipping seedlings into a pycnidiospore suspension prior to transplanting. Twenty nine blackleg epidemics were surveyed over 11 yr. Seedbed transplants (SBT) had been used in 83% of cases. Two cases (7%) had involved direct drilled seedlings (DDS). However, excess seedlings had been transplanted, making DDS epidemiologically equivalent to SBT. Three cases (10%) had involved container-grown seedlings (CGS) grown on mono cropped cabbage lands. Disease occurred in two patterns: in crops grown from SBT and DDS, blackleg occurred down the lines. In all CGS cases, disease occurrence was randomly patterned. In all cases, diseased debris was found in seedbeds and production fields. Disease spread in the field was limited to the two plants on either side of the initially infected plant, 1.3 m or less, suggesting that infection had resulted from splash dispersed pycnidiospores. The disease cycle was mono- or oligo cyclic but not polycyclic. Over a period of 6 yr, cabbage fields of 26 farms were each examined once for cruciferous weeds infected with L. maculans. No viable blackleg lesions were discovered on cruciferous weeds, suggesting that weeds play no role in the local crucifer blackleg pathosystem. A theory is proposed that windows of disease susceptibility open and shut during the different phenological stages of a crucifer's life, and that the susceptibility of different plant organs vary with the phenological state of the plant. It is also postulated that blackleg is a "low sugar disease". Disease incidence was lower in well fertilized cabbage plants than minimally fertilized plants. Organoleptic tests of cabbage cultivars correlated superior flavour and texture in cabbage with a high susceptibility to blackleg. An integrated management strategy is proposed, based on seed treatment with fungicides, the use of container-grown seedlings rather than seedbed transplants, a 3 yr rotation of crucifer lands with non-cruciferous crops, implementation of either deep-ploughing or accelerated biodegradation to eliminate debris, the development of higher levels of horizontal resistance to L. maculans in cruciferous vegetables, application of field fungicides in high risk areas (benzimidazoles or triazoles, or combinations), and the minimization of stress and optimization of host nutrition.
Epidemiology and management of grey leaf spot : a new disease of maize in South Africa.
(1996) Ward, John Michael Julian.; Cairns, Andrew Lawrence Patrick.; Rijkenberg, Fredericus Hermanus Johannes.; Laing, Mark Delmege.
Grey leaf spot is a relatively new fungal disease of maize in South Africa. It has become well established in the province of KwaZulu-Natal, and is capable of reducing grain yields by 20 to 60%. The disease is spreading to neighbouring provinces and countries. This study was conducted to establish solutions to the problem that could be easily implemented by maize farmers. Available literature was reviewed to establish the most appropriate epidemiologically based control measures that might be applicable in South Africa. Field trials were conducted to determine the effects of stubble and conventional tillage practices, cultivar susceptibility, fungicides, the correct time and frequency of fungicide treatment. and the financial benefits of fungicide treatment on grey leaf spot severity. The trials were evaluated for disease severity and grain yields. No commercial hybrids were identified to be resistant to grey leaf spot in the maize hybrid response to grey leaf spot trial. However, subsets of high-yielding hybrids less-susceptible to disease were identified - including PAN 6480, CRN 3584, SNK 2154 and PAN 6578. The most susceptible hybrids were identified to include RS 5206, PAN 6552, A 1849, PAN 6528 and PAN 6140. Fungicides containing carbendazim/flusilazole, were found to be most effective in controlling disease and increased maize yields. Hybrids such as RS 5206 and RS 5232 highly susceptible to disease and showed the highest grain yield response to fungicide treatment, whilst least-susceptible hybrids, such as PAN 6480, had the lowest response. The tillage trial aimed at management practices to reduce grey leaf spot indicated fungicides to be more effective in managing disease than tillage practices aimed at a reduction of initial inoculum. Trials on chemical control of grey leaf spot identified fungicides of the triazole and benzimidazole chemical groups to be effective in controlling disease, but only combination products of these chemical groups, were registered, in support of the pathogen resistance strategy. Products registered were carbendazim/flusilazole, carbendazim/flutriafol and carbendazim/difenoconazole. The frequency and timing of fungicide applications for the control of grey leaf spot in maize studies identified spray treatments initiated when disease had progressed to the basal five leaves and, before the exponential phase of the epidemic, provided the most effective disease control and concomitant high grain yields. Further spray treatments were necessary with early disease infections, in order to provide disease control until crop physiological maturity. The final study on the economic benefits of fungicide treatment of grey leaf spot in maize in KwaZulu-Natal indicated that the highest added yield response was not necessarily the best parameter to justify fungicide treatment. Rather, the expected added profit was a better parameter. In this study the highest added profits were R1 400 ha(-1) from the triple-spray programme in 1993/94 and R439 ha(-1) from a single-spray in 1992/93. The optimum treatment choice depended on the individual's risk-return preferences, which reflect his level of risk-aversion. An integrated approach using tillage practices, crop rotations, hybrids less- susceptible to the pathogen and the judicious use of fungicides is likely to be the most successful in controlling the disease. In the long term, the cornerstone of the integrated approach will be the development and use of hybrids resistant to the disease.
Evaluation of nutritional, chemotherapeutic and educational approaches to manage gastrointestinal nematodes and improve small-scale goat farming.
(2007) Vatta, Adriano Francis.; Rijkenberg, Fredericus Hermanus Johannes.; Worth, Steven Hugh.; Harrison, Leslie J. S.
Small-scale goat farmers from south-western KwaZulu-Natal Province, South Africa, identified gastrointestinal helminths, dry-season feed scarcity and poor reproductive performance as major production constraints and highlighted the paucity of information on goat health and management. The research and extension processes adopted to address these problems comprised on-station experimentation, followed by on-farm validation combined with the participatory dissemination of both study findings and relevant information on goat health care. The approach included the use of the FAMACHA© system to assess anaemia, a characteristic symptom of infection with the gastrointestinal nematode, Haemonchus contortus (Rudolphi, 1803). On-station experimentation indicated that urea-molasses block supplementation during the dry, winter season was a cost-effective option. Unfortunately, when tested on-farm, the value of such supplementation proved inconclusive, possibly due to low block consumption and further research into alternative and palatable protein supplements for goats is suggested. However, tactical anthelmintic treatment with ivermectin effectively reduced faecal egg counts and is recommended, as is concurrent symptomatic anthelmintic treatment, as determined by the FAMACHA© system, since this practice appeared to improve reproductive capacity. Investigations to better adapt the FAMACHA© system to goats is, however, recommended. A flexible training framework was developed with the collaboration of the farmers, providing them with advice on goat health and management. This ‘hands-on’ approach encompassed regular meetings geared to the farmers’ current expertise and exploited the on-farm experimentation as a training vehicle. The process nurtured local farmer ‘champions’, strengthened the extension skills of researchers and technicians and incorporated the development of a Goatkeepers’ animal health care manual. Indications are that the use of such an approach has considerable potential for the development of goat farming. Moreover this process, which is relatively novel for South Africa, is equally applicable to other similar agro-ecological zones. Access to veterinary and agricultural inputs in areas where communal grazing is practised could be vastly improved and a case is made for universities, researchers, extensionists and farmers to collaborate to encourage the long-term sustainable development of these communities.
Infection of Kikuyu grass (Pennisetum clandestinum) by the rust fungus Phakopsora apoda.
(1998) Adendorff, Rosemary.; Rijkenberg, Fredericus Hermanus Johannes.
A rust fungus on kikuyu grass (Pennisetum clandestinum Hochst. ex Chiov.) was identified as Phakopsora apoda (Har. & Pat.) Mains., and was reported for the first time in South Africa. An investigation was conducted into the direct penetration mechanisms employed by the urediospore germlings, using light, scanning and transmission electron microscopy. The infection processes of P. apoda were found to parallel closely those of Phakopsora pachyrhizi. Infection structures of P. apoda produced on artificial membranes are similar to those observed in the host leaf. The binucleate urediospore germinates to form a typically short germ tube, which differentiates an appressorium, delimited by a septum. The two nuclei migrate into the appressorium, where mitosis occurs, resulting in four nuclei in the mature appressorium. Appressoria appear to form preferentially at epidermal cell junctions. When germinated on artificial membranes, germlings differentiate appressoria against microfabricated ridges and on smooth surfaces, possibly with a marginal preference for ridge-associated differentiation (significant at the 5% but not the 1% level of significance). A penetration pore develops in the basal wall of the mature appressorium, over the infection site. A cone-like structure develops around the pore. The basal wall of the appressorium is thinned and electron dense in the vicinity of the cone, and the cone appears to be attached to the appressorial wall by means of a collar of similar, electron-dense material. The membrane-bound cone is elaborately branched, and numerous, electron-dense glycogen-like particles are associated with these cone elaborations. A penetration peg, its walls continuous with an inner wall layer of the cone, penetrates the epidermal cell wall and expands into an intracellular penetration hypha within the host cell. The penetration hole formed by the peg has smooth edges, and there is little deformation of the epidermal cell wall fibrils, indicating a predominantly enzymatic mode of penetration. The penetration hypha traverses the epidermal cell, and emerges into an intercellular space of the mesophyll, narrowing to form a penetration neck at the exit site. Both the penetration peg and the neck, contain multivesicular bodies associated with parallel arrays of microtubules. The intercellular penetration hypha contains an elaborate endomembrane system, and is virtually devoid of glycogen-like particles. Lomasomes often occur in the vicinity of the penetration neck. Signs of host disruption and the formation of papilla-like structures are apparent in mesophyll cells adjacent to the infection site 12 hours after inoculation. A septum delimits the penetration hypha from the primary hypha, which extends further into the mesophyll and forms secondary branches.
Maize root rot in South Africa.
(1995) Hugo, Elbé.; Rijkenberg, Fredericus Hermanus Johannes.
Numerous fungal species are known to infect maize roots and eventually cause rot. The spectrum of fungi differs over localities as well as their relative frequencies. Fungi isolated from discoloured root tissue and root tissue without visible discolouration were classified as root pathogens and root colonizers, based on their isolation frequency from the respective samples. Exserohilum pedicellatum, Macrophomina phaseolina and Fusarium oxysporum were classified as the major root pathogens and Phoma spp., Curvularia spp. and F. chlamydosporum as root colonizers. Fungi classified as root pathogens tended to occur early in the growing season in juvenile tissue as opposed to root colonizers which occurred later. Treatments in an existing long-term water stress trial included no stress (irrigation), normal rainfall, stress until flowering, stress after flowering until mid grain-fill, stress after flowering and total stress. Root pathogens were isolated at higher frequencies in the no stress and normal rainfall treatments than in the total stress treatment. A positive linear relationship between the water stress index and the isolation frequency of M. phaseolina was obtained. Negative, non-linear relationships were, however, recorded for E. pedicellatum, F. oxysporum and F. moniliforme. The effect of tillage practices on disease severity was carried out in field trials at two localities. Tillage practices applied included rip, plough, chisel plough and no-till treatments. Significant differences between isolation frequencies and tillage treatments were recorded for F. oxysporum at Bloekomspruit and Trichoderma spp., Alternaria spp. and M. phaseolina at Mmabatho. Differences in isolation frequency of fungi involved in maize root rot, were determined in a crop rotation system where maize was rotated with soybeans, sunflowers and groundnuts. Crop rotation had a significant effect on the isolation frequency of Thielavia ferricola, E. pedicellatum, F. moniliforme and F. graminearum. The effect of crop rotation, however, seems to be complex since fungi were affected differently in the various treatments.
Studies on Alternaria porri and Stemphylium vesicarium on Allium spp.
(1993) Aveling, Theresa Ann Sheila.; Rijkenberg, Fredericus Hermanus Johannes.; Wehner, F. C.
During surveys in South Africa, Alternaria porri (Ellis) cif. and Stemphylium vesicarium (Wallr.) E. Simmons were found to be destructive seed-borne pathogens of onion (Allium cepa L. ). These two pathogens are also reported on garlic (Allium sativum L.) in South Africa for the first time. The development and morphology of conidiophores and conidia of the two pathogens on the onion leaf surface were examined using scanning electron microscopy. In both pathogens, solitary or fasciculate conidiophores emerged through the epidermis. Bud-like conidial initials were produced singly at the apex of conidiophores. As conidia of S. vesicarium matured, they became oblong to ovoid and densely verrucose. Those of A. porri showed slight growth in width but pronounced elongation. Conidial germination, formation of pre-penetration structures, penetration of the onion leaf surface by A. porri and S. vesicarium, and the subsequent infection process by A. porri, were studied using light, scanning electron and transmission electron microscopy. Conidia of both pathogens usually germinated within 24 h of inoculation, forming several germ-tubes which often terminated in bulbous appressoria produced directly on the epidermal cells or on stomata. Following direct penetration of the outer epidermal cell wall or the stoma, bulbous primary hyphae developed below the appressoria. Secondary hyphae of A. porri developed from primary hyphae and grew within the intercellular spaces, penetrating mesophyll cells. The changes in ultrastructure of infected cells, and of cells in close proximity to secondary hyphae, are described. six fungicides, anilazine, benomyl, carbendazim/flusilazol mixture, procymidone, tebuconazole and thiram, as well as a hotwater soak (50 C for 20 min) and sodium hypochlorite treatment, were evaluated for their efficacy in reducing both pathogens on seed and in culture. The effect of the various treatments on seed germination, and seedling emergence and growth, was determined. None of the treatments eradicated A. porri and S. vesicarium from onion seeds. The hot-water soak proved to be the best treatment for reducing these pathogens, although percentage germination and emergence of onion seeds were reduced when compared to the control.
Studies on Cercospora zeae-maydis, the cause of grey leaf spot of maize in KwaZulu-Natal.
(2000) Caldwell, Patricia May.; Laing, Mark Delmege.; Wallis, Frederick Michael.; Rijkenberg, Fredericus Hermanus Johannes.
In 1983, Latterell and Rossi described grey leaf spot (GLS) of maize (Cercospora zeae-maydis Tehon and Daniels) as "a disease on the move". This pathogen has more than lived up to its reputation. It is estimated to be spreading at a rate of 80-160 km each year, and is recognized as one of the most grain yield-limiting diseases of maize worldwide. The occurrence of the pathogen in the Province of KwaZulu-Natal (KZN), Republic of South Africa (RSA), in 1988, was its first official report from the African Continent. It has since become pandemic, causing grain yield losses of up to 60%. It has spread to other provinces in RSA as well as other African countries, namely Cameroon, Kenya, Malawi, Mozambique, Nigeria, Swaziland, Tanzania, Uganda, Zaire, Zambia and Zimbabwe. It has also been reported to occur in Brazil, China, Columbia, Costa Rica, Mexico, Peru, Trinidad, and Venezuela. The use of soil macro- and micronutrients in the management of fungal plant pathogens is widely documented in the literature. Specific nutrients are known to increase or decrease disease resistance in plants. However, each host-pathogen interaction must be considered on an individual disease basis, together with environmental and soil variables. Although few diseases can be eliminated by a corrective fertilizer regime, the severity of a disease can be reduced by specific nutrients, particularly when used in conjunction with other cultural practices. However, the economic implications, and not grain yield alone, of different control measures should be considered; i.e., farmers must compare the expected added gross margin ha -1 (added income minus added costs) with the potential variability in expected added gross margin ha -1 (upper and lower limits) of each treatment when deciding on which fertilizer applications and/or fungicide treatments to use. Literature reviews were undertaken on both GLS and the use of soil nutrients to control fungal plant pathogens to provide the necessary background technical information in order to conduct research under local conditions, and to assist in interpretation of results of experiments. Nutrient trials to control GLS were conducted at two sites in KZN, i.e., Cedara (1995/96, 1996/97 and 1997/98) and Ahrens (1995/96). Research at Cedara showed that with increased applications of nitrogen (N) at 0, 60 and 120 kg N ha -1 and potassium (K) at 0, 25, 50 and 150 kg K ha -1, leaf blighting occurred earlier, and final percentage leaf blighting and the standardized area under disease progress curve were higher. The Ahrens trial also showed that with increased applications of N (0, 60, 120 and 180 kg N ha -1) and K (0, 50, 100 and 150 kg K ha -1), there were also increases in final percentage leaf blighting. Increasing phosphorus levels of 0, 30, 60 and 120 kg P ha -1 did not have any effect on final percentage leaf blighting. The application of systemic fungicides to GLS-susceptible maize was highly effective in controlling GLS and increasing grain yields substantially with increased N and K applications. In the non-fungicide treated plots, grain yields did not increase with increased applications of K in all three years of the trial. This was probably because grain yield response, which should have occurred at higher K applications, was reduced by increased GLS severity. Similarly, grain yields did not increase significantly with N application in 2 of the 3 years of the trial. At Cedara, non-fungicide treated maize produced a financial loss of -R165 and -R48 with 25 and 50 kg K ha -1 respectively, relative to 0 kg K ha -1. However, increasing N applications resulted in increasing grain yields, and added gross margins of R714 ha -1 and R536 ha -1 with applications of 60 and 120 kg N ha -1, respectively. The drop in added gross margin at 120 kg N ha -1 was probably because of increased GLS levels at higher fertiliser rates, resulting in reduced grain yields. In fungicide treated maize, added gross margin relative to 0 kg K ha -1 increased from R851 to R1212 ha -1. However, there was a loss of -R133 ha -1 in added gross margin relative to 0 kg N ha -1 at 60 kg N ha -1 as increased grain yields did not offset the added cost of N fertilizer and fungicide applications. At 120 kg N ha -1 added gross margin relative to NO was R423 ha -1. Highest grain yields and gross margins in fungicide treated maize were obtained with 120 kg N ha -1 and 150 kg K ha -1, as expected. However, in non-fungicide treated maize, highest grain yields and gross margins were obtained using 60 kg N ha -1 and 50 kg K ha -1. This was because of higher GLS severity at the higher N and K application rates. Yields of wheat grown in soils with residual fertilizers after non-fungicide treated maize were higher (4.21 ha -1) compared to yields (3.61 ha -1) grown on residual fertilizers after maize that had been sprayed to control GLS. This was probably as a result of GLS reducing the photosynthetic area of maize leaves, causing premature death with a concomitant reduced uptake of nutrients by roots. This resulted in higher residual levels of fertilizers in soils where fungicide applications were not used to control GLS on maize compared to soils planted with maize where GLS was controlled through the application of fungicides. In KZN there are approximately 350,000 small-scale farmers. The same diseases that affect commercial agricultural production also affect the small-scale farmer, the major difference being in the methods of disease control employed. At the commercial level, most farmers rely on the use of agro-chemicals, which are often not available to the small-scale farmer due to the relatively high cost of agro-chemicals, application methods, and the non-availability of products in the rural areas. The level of illiteracy of the small-scale farmer may also inhibit the use of agro-chemicals. In many African countries, the per capita consumption of maize may be as high as 100 kg per year. Production of cereals in Africa has fallen in the past 25 years. This, together with yield reductions of maize caused by GLS, is likely to contribute to an even greater food deficit in many African countries. At present, low soil fertility and pH levels are a problem among small-scale farmers both in the RSA and other parts of Africa. In the RSA, government policy is to increase maize production by small-scale farmers through improved agronomic methods, including increased fertilizer application. Appropriate and affordable rotations and other improved agronomic practices need to be developed and promoted to ensure food security and sustainable systems for smallscale farmers. The results from the nutrient trials presented in this thesis have practical applications for the small-scale farmer who does not have the option of controlling GLS through the use of agrochemicals. The small-scale farmer will be able to attain a maximum gross margin from his maize crop by applying 60 kg N ha -1 and 50 kg K ha -1, if no fungicides are applied. However, comparative analyses of manure showed that a small-scale farmer would have to apply 1-3 tonnes of manure in order to achieve similar nutrient levels - a procedure that would be impractical. Comparative financial analyses of aerial and knapsack fungicide applications showed that it would be uneconomical for the small-scale farmer to apply fungicides using a knapsack sprayer. A simple spreadsheet has been created to help farmers make the best choice of N (0, 60 or 120 kg N ha -1) and K (0, 25, 50 or 150 kg K ha -1) and the number of fungicide application (O, 1, 2 or 3). This will eliminate the guesswork needed for farmers to maximize gross margins, based on a specific amount of money available. The resistance expressed by different hybrids on conidial germination of C. zeae-maydis at varying temperatures, desiccation periods and interrupted dew periods was investigated using the susceptible ZS 206 and the less susceptible SC 625 maize cultivars. Germination of conidia was maximized at 28°C on both cultivars by 48 hr with ZS 206 showing 100% germination, in contrast to only 63% germination in SC 625. As the number of days (1-5) of desiccation increased following inoculation, germination decreased from 100 to 47% in ZS 206 and from 62 to 0% in SC 625, respectively. The observation that C. zeae-maydis is able to tolerate unfavourable conditions and resume germ tube growth when favourable conditions return was confirmed in interrupted dew period studies. There was no change in percentage germination after 48 hrs., when plants were subjected to interrupted dew periods of 2-36 hrs, following a 6 hr period at 95-100% RH at 28 °C in a dew chamber. However, germination was lower (64%) on SC 625 than ZS 206 (90%). The wider range of temperature conditions favourable for conidial germination of ZS 206, and the fact that it was less affected by desiccation and interrupted dew periods than SC 625, could account for the different susceptibility levels of these two hybrids to GLS. Peak daily conidial catches were found to be between 1200 and 1400 hrs when temperatures and vapour pressure deficits were highest and leaf wetness lowest. Multiple regression analyses identified high evaporation over a 24 hr period, low temperatures over a 48 hr period and wind over a 72 hr period as the weather variables most strongly associated with high conidial releases. Rain, high vapour pressure deficit values and temperatures between 20-30 °C with leaf wetness over a 72-day period, together with prolonged high evaporation over a 48 hr period were identified as limiting factors in conidial release. These results indicate that temperatures (< 20 °C) and moisture 24-48 hrs prior to release is required for production of conidia. However, dry air and leaf surfaces are required for conidia to break off conidiophores at the point of attachment, i.e., a hygroscopic process is involved in release of conidia in C. zeae-maydis. In general, the process of conidiogenesis in C. zeae-maydis is similar to that observed on C. beticola. Successive formation of conidia on the same conidiophore are in accord with previous observations on C. zeae-maydis. Conidial measurements are also similar to other taxonomic descriptions of C. zeae-maydis. Hyphae aggregate in the substomatal cavity and give rise to fascicles of 1-2 septate conidiophore initials which emerge through the stoma. A single, aseptate conidium develops from the conidiogenous cell of the conidiophore initial. Extension growth of the conidiogenous cell from the base and one side of the terminal conidium, leads to the lateral displacement of the conidium on the conidiophore. After conidial secession, the conidiophore continues to grow, producing a second conidium from the conidiogenous cell at the apex of the extended conidiophore. This sympodial and successive proliferation of the fertile conidiogenous cell results in the formation of a characteristic 1-3 geniculate, occasionally 4, conidiophore, bearing a single conidium at each apex. This body of research has added information that was previously missing in the lifecycle of C. zeae-maydis. However, this additional information has, in turn, led to other yet unanswered questions which need to be addressed in the future, particularly under southern African conditions. A thorough knowledge and understanding of the epidemiology of this pathogen can result in more effective control strategies with increased yields for both commercial and small-scale farmers in KZN.
Studies on ear rot and grey leaf spot of maize in South Africa.
(1997) Nowell, David C.; Rijkenberg, Fredericus Hermanus Johannes.; Laing, Mark Delmege.
In recent years there have been economically important epidemics of both Stenocarpella ear rot and grey leaf spot (GLS) in South Africa. These epidemics have adversely affected the grain yield and quality of the maize harvested. Maize researchers and breeders have had to re-assess the importance of maize disease in South Africa and make the necessary adjustments to their programmes. Literature reviews were undertaken on both Stenocarpella ear rot and GLS to provide the necessary background of technical information to conduct research under local conditions into these disease problems, and to assist in interpretation of results of experiments. A novel method of inoculating milled Stenocarpella infected ears into the whorls of maize plants (about 2 weeks before 50% anthesis) was developed to provide consistent inoculum pressure and increased ear rot. This inoculation method was practical, efficient, reliable, consistent and cheap to implement. Commercial organisations could use this inoculation method to inoculate a large number of plants per day, allowing for improved screening of breeding material and hybrids. Ear rot assessment methods, and researchers' ability to assess ear rot, were tested under South African conditions. The accuracy of the different methods tested varied considerably, particularly when there was a high level of ear rot that could not be seen without shelling the grain. Each method could be used in a maize breeding programme, depending upon the desired levels of accuracy and time taken using the given method. Researcher's ability to assess ear rot varied considerably and accuracy was correlated with the number of years experience in maize research. Grain colour affected the researcher's ability to accurately assess ear rot severity. Yellow-grained maize was more difficult to assess for ear rot than white-grained maize. Hybrid response to Stenocarpella ear rot infection was difficult to interpret owing to a significant interaction with the environment. Hybrid ear rot response was non-linear in nature. Normal methods of presenting disease data and classifying hybrids in resistance response categories were not successful. Non-linear regression analysis has to be used to do this. However, it is important that ear rot data be presented in a way that farmers can utilise the information. Pre-flower stress predisposes maize hybrids to ear rot infection. Hybrids that normally exhibited good levels of resistance to Stenocarpella ear rot may become severely colonised if drought stress occurs in the four weeks prior to flowering. This environmental interaction makes ear rot resistance breeding and the interpretation of results difficult. The incidence of maize ear rot was widely considered to increase with increased plant density. Experiments over three seasons in South Africa have shown that is not true under certain environmental conditions. In specific hybrids, plant densities of less than 50 000 plants ha"1 exhibited a higher incidence and severity of ear rot than plant densities greater than 50 000 plants ha(-1). The hybrids that usually responded in this manner were more susceptible to ear rot than the other hybrids. Generally, ear rot increased with increased plant densities over 50 000 plants ha(-1). The mechanisms and reasons for this could not be determined. Fungicide trials and regression analysis of hybrid yield trials over a two years period, at two locations in KwaZulu-Natal, showed that grain yield losses due to GLS infection were at least 13%. Severity of GLS was consistently higher at Cedara than at Greytown. Economic losses at Cedara ranged from Rl 919 - R2 278 ha(-1) and at Greytown from Rl 554 - Rl 726 ha(-1). Predicted hybrid losses ranged from R836 - R2 621 ha(-1) (13% - 37%), depending upon the level of inherent GLS resistance. Hybrid response to Cercospora zeae-maydis infection was linear in nature and hybrids could be categorised into response categories. Large differences in GLS resistance could be found between commercial hybrids. However, the current levels of GLS resistance in hybrids does not eliminate yield loss under high GLS inoculum levels, and fungicide application was economically justified on most hybrids. Newly released hybrids show increased levels of GLS resistance. The application of systemic fungicides to GLS-susceptible maize was highly effective in controlling GLS and increasing yield substantially. The most effective fungicides belonged to the triazole and benzimidazole group of fungicides. Protectant fungicides were not as effective as systemic fungicides. Copper-based fungicides were phytotoxic to maize in two seasons and at both locations. Fungicide mixtures of the two groups active against GLS are being used on commercially. The effectiveness of fungicides did not vary over location or hybrids, but was influenced by inoculum pressure. Effective control strategies have been implemented to control both Stenocarpella ear rot and GLS in South Africa. Crop rotation, the selection of the more ear rot and GLS-resistant hybrids, and the judicious use of fungicides has reduced the levels of both diseases to manageable levels. An integrated control strategy is needed to control these diseases and efforts are being made to educate farmers to this effect. Maize pathological research now enjoys a greater emphasis than it did in the early-1980s.
Studies on Puccinia recondita f. sp. tritici with special emphasis on adult plant resistance in wheat.
(1986) Pretorius, Zacharias Andries.; Rijkenberg, Fredericus Hermanus Johannes.; Wilcoxson, Roy Dell.
Leaf rust (Puccinia recondita f.sp. tritici) of wheat (Triticum aestivum) was widespread in South Africa during 1983, 1984 and 1985 and often reached epidemic levels, especially on autumn-sown spring wheat in the Cape Province. Nine physiologic races were identified during the study period. The most common race was avirulent to the leaf rust differential genes Lr3a, 3bg, 3ka, 11, 16, 20 and 30 and virulent to Lr1, 2a, 2b, 10, 14a, 15, 17, 24. Resistance genes Lr9, Lr19, Lr21 and Lr26 were effective to all isolates tested. Evaluation of wheat genotypes for components of resistance, viz. infection type, latent period, number of uredinia and uredinium size, revealed three phenotypic reaction classes. The first group exhibited negligible resistance, the second was susceptible or moderately susceptible as seedlings but resistant as adult plants while the third group was resistant at all growth stages tested. Adult plant resistance was expressed by hypersensitive or non-hypersensitive reactions and the combination of components conditioning resistance varied. Adult plant resistance conferred by gene Lr22a was characterized by a long latent period, small uredinia, reduced sporulation and an absence of a differential interaction between components of resistance and different races of Puccinia recondita f.sp. tritici. Numbers of uredinia on flag leaves of RL6044 (Lr22a) were equal to those of a susceptible check, Line E. Lr22a was inherited as a partially recessive gene in crosses with Zaragoza and SST33. Assessment of latent period, number of uredinia and infection type in F4 and FS families homozygous for Lr22a and derived from crosses between RL6044 and Zaragoza or SST33, revealed significantly different levels of resistance between families. Differences were attributed to other genes modifying the expression of Lr22a. Adult plant resistance of Era, Glenlea, RL6044 and sinton was expressed prior to the fifth-leaf stage. Latent period increased and number of uredinia decreased as each wheat matured. While the latent period of the flag, flag-l and flag-2 leaf was similar within Era, Glenlea and RL6044, differences between these genotypes occurred. The latent period of flag leaves of Sinton was shorter than that of the two lower leaves. Significantly fewer uredinia developed on the flag-2 leaf of Glenlea. A reduction in temperature from 21 C to 15 C significantly increased latent period in Era, Glenlea and RL6044, and also restricted uredinium size on flag leaves of RL6044. The adult plant resistance of Glenlea crossed with Line E was conferred by two partially recessive genes. Additionally, F2 to FS progenies of this cross eXhibited high levels of hypersensitive seedling resistance at 29 - 31 C to certain isolates. The latter resistance was not conferred by Lr1 or by the LrT2 gene for mature plant resistance in Glenlea. The high-temperature expression of resistance could be due to a second gene for adult plant resistance or to a previously undetected seedling gene.
Studies on the expression of resistance of Coffea selections to Hemileia Vastatrix.
(1990) Coutinho, Teresa Ann.; Rijkenberg, Fredericus Hermanus Johannes.
Physiological races of Hemileia vastatrix in southern Africa were identified. Prevalent races were I (2.2%), II (88.9%) and III (2.2%). Six samples could not be identified. Twelve biotypes of race II were distinguished. In some cases, the biotypes only occurred in specific regions. It was established, using fluorescence microscopy, that, in some cases, the percentages of germinated urediospores that did not form appressoria, appressorium formation over stomata, and aborted appressoria, were significantly different between susceptible and resistant selections of the host, and non-hosts. The sequence of events leading to successful infection was investigated using scanning electron microscopy (SEM). When a stoma is encountered by a germ tube tip a uniquely shaped appressorium forms over one end of the stomatal slit. A distinct appressorial foot is wedged within the stomatal vestibule. In coffee, a torpedo-shaped substomatal vesicle initial (SSVI) develops bilaterally from the apex of the infection wedge, While in bean, the infection wedge protrudes into the substomatal chamber. The substomatal vesicle (SSV), at 48 hours post inoculation (hpi) is anchor-shaped. Haustorial mother cells are formed on stubby primary infection hyphae which curve back onto subsidiary cells. No differences in appearance of these structures were noted between resistant and susceptible coffee selections. A much-branched mycelium ramifies through the intercellular spaces of the mesophyll cells 96hpi. In bean, the SSV began to collapse 48hpi. Bayfidan ® only slightly suppressed fungal development on the leaf surface. However, within susceptible tissue, this systemic fungicide had an effect on the morphology of the fungus. Extracellular material accumulated on the SSVI and the SSV. The SSV appeared swollen, and disruptions in the vesicle wall was noted. The discovery of teliospores on locally infected trees led to a SEM study on their structure, development and germination. Infection structure formation on the leaf surface, latent period, reaction score and urediosorus concentration differed between susceptible coffee leaves of different ages. Generally, mature leaves are more susceptible than very young or old leaves. A range of fungicides, mainly systemics, were tested in the field on naturally infected coffee trees. Various epidemiological and climatic factors influence rust development in the field. The role of these factors at the fungicide site and in commercial coffee-growing regions of southern Africa was evaluated.
Studies on the expression of resistance to stem rust of wheat caused by Puccinia graminis f.sp. tritici.
(1991) Lennox, Cheryl Lynne.; Van Staden, Johannes.; Rijkenberg, Fredericus Hermanus Johannes.
The endogenous cytokinin levels of healthy primary leaves and seeds of a stem-rust susceptible wheat cultivar Little Club were compared with those of Little Club containing the stem rust resistance gene Sr25. Use was made of paper, column and high performance liquid chromatography techniques to separate the endogenous cytokinins in the plant material, and the soybean callus bioassay was used to test for cytokinin-like activity of the chromatography fractions. Leaf material of the resistant Little Club Sr25 had a higher level of total cytokinin activity than Little Club, whereas seed material of Little Club Sr25 did not always have higher levels of cytokinins than Little Club. A number of cultivars would have to be tested before the usefulness of cytokinin levels as an indicator of resistance could be determined. The development of urediospore-derived infection structures of Puccinia graminis f.sp. tritici in wheat, barley, sorghum and maize was examined by scanning electron microscopy (SEM). Infection on and in the four species followed a similar pattern up to, and including, primary infection hyphae formation. In wheat, barley and maize, when a primary infection hypha abutted onto a host epidermal cell, a septum was laid down delimiting a primary haustorial mother cell (HMC); primary HMCs did not form in sorghum. Secondary infection hyphae arose on the substomatal vesicle side of the primary HMC septum; infection did not progress further in maize, but in wheat and barley secondary infection hyphae branched, and proliferated intercellularly forming the fungal thallus. Secondary HMCs were delimited when an intercellular hypha abutted onto host cells. In all four species atypical infection structures were also observed. In an attempt to determine the timing and expression of stem rust resistance gene Sr5, infection structure development of Puccinia graminis f.sp. tritici race 2SA2 in a resistant line (ISr5Ra) and a susceptible line (ISr8Ra) was compared quantitatively using a fluorescence microscopy technique. The results indicated that there were no significant differences in numbers of specific infection structures observed in the two near-isogenic lines up to, and including, 48 hpi, by which time race 2SA2 had successfully formed secondary HMCs in both lines.
Studies on the pathogenicity of Puccinia graminis f. sp. Tritici and the nature of host resistance in wheat.
(1986) Le Roux, Jacobus.; Rijkenberg, Fredericus Hermanus Johannes.
This study was undertaken to investigate the pathogenic variation of Puccinia graminis f.sp. tritici in South Africa and to determine the nature of host resistance to this fungus. Seventeen pathotypes of wheat stem rust were identified in 1981 and only one during 1982 and 1983. The 1984 and 1985 surveys provided six and nine pathotypes respectively. These included two new pathotypes 2SA100 and 2SA101, both virulent for Sr24. No avirulence was detected for wheat stocks having Sr7a, 8a,10,11,14,16,18,19,20 and 28. No virulence was found on wheats with Sr8b,13,21,22,25,26,27,29,31,32,35,dp2 and Tt3. Data suggested that the pathogenicity pattern in South Africa is strongly influenced by the resistance genes present in wheat cultivars. Pathogenic studies of pathotypes 2SA100 and 2SA101 cultivars demonstrated that both possessed increased virulence for Sr24 and appeared to be mutations of earlier types. Seedling and adult plant response studies indicated genetic vulnerability in 60% of the locally cultivated cultivars. Nine of the 23 recommended cultivars possess Sr24, either alone or in combination with other resistance factors. Pathotypes 2SA100 and 2SA101, or races similar to these, constitute a major threat to wheat production in Southern Africa since they combine increased virulence with aggressiveness and good survival ability. Genetic studies showed that resistance was conferred by one dominant gene in each of Belinda (Sr5), SST33 (Sr9e), SST66 (Sr9e), SST102(Sr24), Gamka (Sr24), Gamtoos (Sr31), Karee and Betta. Two partially dominant genes were identified in Wilge, Gouritz (Sr36+) and SST23 (Sr24+). Resistance in Palmiet was conferred by one dominant, (Sr24) and one recessive gene, whereas one recessive and one dominant gene operated in Molen. Tugela carried three partially dominant genes, one of which was positively identified as Sr5. Palala also has three unidentified dominant genes. The study determined that inheritance of resistance in 15 South African cultivars was relatively simple with little genetic diversity. Three spring wheat cultivars, SST44, Palmiet and Gamtoos were used to study the quantitative aspects of specific resistance to P. graminis tritici. Averaged over three cultivars, latent period was extended by 57% and uredinium density was reduced by 29% relative to the susceptible check Morocco. Latent period appeared to be a race-nonspecific resistant component of all three cultivars. This study indicated that the variation in uredinium density associated with specific resistance was similar to that reported by other workers for compatible host pathogen interactions.
Studies on the resistance of wheat and maize to fungal pathogenesis.
(1990) Van Asch, Michiel Alfred Jan.; Rijkenberg, Fredericus Hermanus Johannes.
17-Day-old seedlings of winter-wheat cultivar SST25 were inoculated with an avirulent race of wheat leaf rust, Puccinia recondita f.sp. tritici. After various time intervals the plants were reinoculated with a virulent wheat leaf rust race. No change in latent period or infection type was observed. However, the infection frequency was reduced by approximately 60 per cent. The phytotoxic effects of three mycotoxins of Fusarium spp. (fumonisin B(1)(FB)1) , moniliformin and T-2 toxin), and pathotoxin extracts of Exserohilum turcicum (HT-toxin) and Stenocarpella macrospora (SM-toxin) were studied using callus from the scutella of immature cobs of maize, Zea mays. The callus was grown on modified MS medium containing either 0, 0.1, 1.0, 10, or 100 mg (or ml) toxin per litre. For SM-toxin the concentrations used were 0, 0.01, 0.1, 1.0 or, 10 ml/l. Mass increase of callus on medium containing FB 1, moniliformin, T-2 toxin, and HT-toxin decreased as the concentration of toxin increased, resulting in a significant reduction at the highest toxin level. SM-toxin callsed a slight reduction in mass at 0.01 ml/l, but stimulated growth at 1.0 ml/l. At 10 ml/l a significantly lower callus mass increase was recorded. Transmission electron microscopy studies of FB 1-treated callus showed an increased level of activity in the toxin-treated cells resulting in thicker cell walls, occurrence of starch grains and phenolic substances, when compared to the control. The mitochondria of callus cells were affected by SM-toxin, and starch was found in all toxin treatments. When transferred to toxin-free medium after treatment with FB 1, a complete recovery of the callus occurred at all toxin levels but the highest, although regrowth occurred at this level. Callus treated with SM-toxin retained the same growth rate as during the toxin treatment, and it can be concluded that the toxin has a permanent effect on the growth rate of callus. Maize seedling leaves, injected with a 10g/l FB(1)-solution at the stalk base, showed necrotic areas and chlorotic flecks. The toxin-treated plants were stunted and occasionally produced side shoots. S. macrospora-susceptible and -resistant seedlings, injected in a similar fashion with SM-toxin, gave a different response to the toxin. Susceptible plants were affected by the toxin, while no effects were observed in resistant plants.
A study on the ultrastructure and control of the gladiolus rust pathogen, Uromyces transversalis.
(1988) Ferreira, Johan Francois.; Rijkenberg, Fredericus Hermanus Johannes.
The substomatal vesicle of Uromyces transversalis develops in five distinct stages after the formation of a single infection peg in the host plant Gladiolus spp. The primary hyphae of the substomatal vesicle orientate at right angles (rather than at acute angles or parallel) to the stomatal slit and the parallel veins of a gladiolus leaf or to those of the non-host, Zea mays. The transverse orientation of Uromyces transversalis uredia is apparently a genetically controlled phenomenon. The two primary hyphae normally develop asynchronously on opposite sides of the substomatal vesicle, and, after the formation of a haustorium mother cell, secondary hyphae branch off. These subsequently form basal cells each of which give rise to one or more holoblastic protruberances on its distal surface. A septum delimits the protruberance from the basal cell to form a urediospore initial, which in tum elongates and, by septum formation, forms a pedicel and a urediospore. The urediospore is seceded mechanically through the process of schizolysis. The pedicel of a spore thus formed remains on the basal cell and becomes a collar around the next protruberance. The basal layer of the two-layered septum, that delimited the pedicel from the basal cell, grows out to form the wall of the subsequent protruberance, in the process rupturing and laterally displacing the terminal septal layer. A new basipetal septum forms to delimit the subsequent urediospore initial. Therefore, successive urediospores are formed enteroblastically and give rise to a series of basipetal collars. Cultivars, naturally occurring species and breeding lines of gladiolus were evaluated for rust resistance. The cultivars showed no sign of resistance, whereas the populations of wild species and breeding lines had similar resistance profiles. The species G. daleni and G. tristis var. tristis showed promise for breeding. Infection of these species caused an almost immune, fleck reaction. The resistance of the species G. daleni was manifested by the abortion of primary hyphae prior to the formation of the haustorial mother cell. The intercellular hyphae of gladiolus rust only partially adhered to the mesophyll cell of the resistant host. The haustorial apparatus that did form in the mesophyll cells in the resistance reaction was inhibited at various stages of its development. The rust disease could be controlled chemically by either bitertanol or triadimefon. Triadimefon, however, caused a shortening of the internodes of the flower spike. The early development of U. transversalis infection structures in bitertanol-treated leaves was inhibited at, or shortly after, substomatal vesicle development. A number of mature substomatal vesicles with intercellular hyphae, however, did develop in the bitertanol-treated leaf tissue, probably because this fungicide undergoes limited translocation in the gladiolus lamina.
An ultrastructural study on rusts with special reference to Puccinia sorghi on Oxalis corniculata.
(1973) Rijkenberg, Fredericus Hermanus Johannes.; Truter, Susarah J.
No abstract available.
Ultrastructure, cytochemistry and immunocytochemistry of the interaction between wheat (Triticum aestivum) and leaf rust (Puccinia Recondita f.sp. tritici)
(1996) Hu, Guanggan.; Rijkenberg, Fredericus Hermanus Johannes.
The development of infection structures, derived from urediospores of Puccinia recondita f.sp. tritici in near-isogenic lines of susceptible and resistant wheat, and in non-hosts, viz. maize, oat, sorghum and barley, was examined by fluorescence microscopy and scanning electron microscopy (SEM). The infection structure formation on and in five cereal species follows a similar pattern. In sorghum, fungal development is arrested at the stage of substomatal vesicle formation, while, in maize, most fungal structures collapse during the stage of primary hypha development. On the other hand, in wheat, barley and oat, the fungus forms many branched infection hyphae and haustorial mother cells. There were no significant structural and numerical differences in infection structure development between susceptible and resistant wheat lines. The ultrastructure of intercellular hyphae and D-haustoria of P. recondita f.sp. tritici, and the host response to haustorial invasion, was investigated. The intercellular hyphae share common characteristics with other uredial stage rust fungi. Anastomosis was observed between intercellular hyphae. Two nucleoli were frequently observed in a single nucleus in the haustorium, indicating possible nuclear fusion between the two nuclei in D-haustoria of this fungus. The close association of host organelles, such as the nucleus, Golgi bodies, endoplasmic reticulum , vesicles and mitochondria, with the developing haustorium, was described. The investigation of urediospore formation of P. recondita f.sp. tritici on wheat leaves by SEM and transmission electron microscopy (TEM) showed that one or more protuberances arise sympodially from several different loci on the distal surface of a basal cell, each protuberance developing into a urediospore. At the same site at which one urediospore formed previously, at least one other urediospore initial can form subsequently. A study of the cytochemistry of the interaction between wheat and P. recondita f.sp. tritici, using various enzyme- and lectin-conjugated gold probes, was conducted. This research provided additional information on the nature and composition of the walls of fungal hyphae, the haustorial mother cell, the haustorial neck, the haustorial body and the extrahaustorial matrix. Cellulose, the major component of the host cell wall, was not detect.ed in the extrahaustorial matrix and in the host tubules associated with the invaded haustorium. The composition of walls of the haustorial body of P. recondita f .sp. tritici appears to change as the haustorium matures. The study identified the existence of mannose/glucose, galactose, N-acetylgalactosamine and fucose residues in the extrahaustorial matrix. An antiserum raised against the purified 33 kDa wheat β-1 ,3-glucanase was used to investigate the subcellular localization of the enzyme in P. recondita f.sp. tritici-infected wheat leaves by means of a post-embedding immunogold labelling technique. In the compatible interaction, β-1,3-glucanase was demonstrated to accumulate predominantly in the haustorial wall and extrahaustorial matrix. In the incompatible interaction, strong labelling for β- 1 ,3-glucanase was found in host cell wall appositions, extracellular matrix in the intercellular space, and in electron-dense structures of host origin which only occurred in the incompatible interaction. Using anti-zeatin riboside and anti-isopentenyladenosine antibodies in post-embedding immunocytochemical procedures, cytokinins were localized at the ultrastructural level in P. recondita f .sp. tritici-infected wheat leaves. The sites where cytokinins accumulate were not significantly different between the compatible and incompatible interactions. The cytokinins are mainly present in the fungal cytoplasm of the intercellular hyphal cell, the haustorial mother cell, the haustorial body and extrahaustorial matrix, indicating that cytokinins, primarily of fungal origin, are associated with the nutrient translocation in this host-fungus interaction.