Browsing by Author "Putter, Christoffel Antonie Johannes."

Now showing 1 - 2 of 2

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.