Browsing by Author "Tesfagiorgis, Habtom Butsuamlak."

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Studies on the biocontrol of seedling diseases caused by Rhizoctonia solani and Pythium sp. on sorghum and tef.
(2003) Tesfagiorgis, Habtom Butsuamlak.; Laing, Mark Delmege.; Caldwell, Patricia May.
Rhizoctonia solani and Pythium spp. are aggressive soil-borne fungal pathogens responsible for seed rot and seedling damping-off of many crops. With increased environmental and public concern over the use of chemicals, biological control of these diseases has been attracting more attention. However, success with this strategy depends on the development of effective antagonists, which requires repeated in vitro and in vivo tests. Bacillus spp. were isolated from a soil sample obtained from a field where sorghum and tef had been grown for at least two years. Potential Bacillus isolates were screened for their ability to inhibit in vitro growth of R. solani and Pythium sp. Among 80 isolates tested, endospore forming Bacillus spp. H44 and H51 gave highest antifungal activity against the two test-pathogens in three consecutive tests. Results demonstrated that both H44 and H51 have potential as biocontrol agents against diseases caused by these two pathogenic fungi. The interaction between three isolates of Trichoderma (T. harzianum Eco-T, Trichoderma spp. SY3 and SY4) and Pythium sp. were investigated using in vitro bioassays together with environmental scanning electron microscopy (ESEM). Visual observation on the dual culture tests revealed that hyphal growth of Pythium was inhibited by these antagonists soon after contact between the two organisms within 3-4 days of incubation. The ESEM investigations showed that all three isolates of Trichoderma grew toward the pathogen, attached firmly, coiled around and penetrated the hyphae of the pathogen, leading to the collapse and disintegration of the host's cell wall. Degradation of the host cell wall was postulated as being due to the production of lytic enzymes. Based on these observations, antibiosis (only by Eco-T) and mycoparasitism (by all three isolates) were the mechanisms of action by which in vitro growth of Pythium sp. was suppressed by these Trichoderma isolates. The reduction of seedling diseases caused by R. solani and a pythium sp. were evaluated by applying the antagonists as seed coating and drenching antagonistic Bacillus spp. (B81, H44 and H51) and Trichoderma (T. harzianum Eco-T and Trichoderma spp. SY3 and SY4). On both crops, R. solani and Pythium sp. affected stand and growth of seedlings severely. With the exceptions of H51, applications all of isoltes to seeds reduced damping-off caused by R. solani in both crops. Application of Eco-T, H44 and SY3 to sorghum controlled R. solani and Pythium sp. effectively by yielding similar results to that of Previcur®. On tef, biological treatments with Eco-T and SY4 reduced seedling damping-off caused by R. solani and Pythium sp., respectively, by providing seedling results similar to the standard fungicides, Benlate® and Previcur®. Most other treatments gave substantial control of the two pathogens on tef. Overall, Bacillus sp. H44 and T harzianum Eco-T were the best biocontrol agents from their respective groups in reducing damping-off by the two pathogens. In all instances, effects of application method on performance of biocontrol agents and adhesive on emergence and growth of seedlings were not significant. A field trial was conducted at Ukulinga Research Farm at the University of Natal, Pietermaritzburg, South Africa, to determine efficacy of biological and chemical treatments on growth promotion and reduction of damping-off incited by R. solani and Pythium sp., and to evaluate the effects of a seed coating material, carboxymethyl cellulose (CMC), on seedling emergence and disease incidence. Seeds of sorghum and tef were treated with suspensions of antagonistic Bacillus H44 or T harzianum Eco-T, or sprayed with fungicides, Benlate® or Previcur®. Application of Benlate® and Previcur® during planting significantly increased the final stand and growth of sorghum seedlings. Seed treatments with both H44 and Eco-T substantially controlled damping-off caused by Pythium, resulting in greater dry weights of seedlings than the standard fungicide. However, they had negative effects when they were tested for their growth stimulation and control of R. solani. The CMC had no significant effect on germination and disease levels. These results showed that these antagonists can be used as biocontrol agents against Pythium sp. However, repeated trials and better understanding of the interactions among the antagonists, the pathogens, the crop and their environment are needed to enhance control efficiency and growth promotion of these antagonists. Some of these biocontrol agents used in this study have the potential to diseases caused by R. solani and Pythium sp. However, a thorough understanding of the host, pathogen, the antagonist and the environment and the interactions among each other is needed for successful disease control using these antagonists.
Studies on the use of biocontrol agents and soluble silicon against powdery mildew of zucchini and zinnia.
(2008) Tesfagiorgis, Habtom Butsuamlak.; Laing, Mark Delmege.
Powdery mildew (PM) is an important foliar disease of many crops, occurring under both greenhouse and field conditions. The application of biological control and soluble silicon (Si) against PM has received increasing acceptance as a result of increased environmental and public concern over the use of fungicides for disease management, and because many key fungicides are no longer effective because of resistance problems. However, success with these control options depends on the development of effective antagonists and understanding how best to use Si in agriculture. Potential antagonists of PM were isolated from naturally infected leaves of different plants. A total of 2000 isolates were tested in a preliminary screening on detached leaves of zucchini. The best 30 isolates showing consistent results were further tested under greenhouse conditions for their efficacy against PM of zucchini. In a greenhouse trial, 23 isolates provided disease control to levels of 30 to 77%. Application of 29 isolates resulted in significant reductions in values of area under disease progress curve (AUDPC). The best five isolates were identified as Clonostachys rosea (Link) Schroers, Samuels, Seifert & Gams (syn. Gliocladium roseum) (Isolate EH), Trichothecium roseum (Pers.) Link (syn. Cephalothecium roseum) (Isolate H20) and Serratia marcescens (Bizio) (Isolates B15, Y15 and Y41). Three adjuvants (Break-ThruR (BK), PartnerR (PR) and Tween-80R (T-80)) were compared for their ability to improve efficacy of spray application of silicon (Si) and biocontrol agents (BCAs) against PM. Both BK and PR improved the efficacy of Si significantly (P < 0.05). Microscopic studies showed that BK affected PM fungi directly and enhanced the deposition of BCAs on the pathogen. Break-ThruR was only toxic to the pathogen mycelia when used at > 0.25 m. .-1, but phytotoxic to zucchini plants when used at > 0.45m. .-1. However, it did not affect the c.f.u. of bacterial BCAs. Use of BK at 0.2-0.4 m. .-1 can be recommended to assist spray application of Si (at 750 mg .-1) or BCAs for improved control of PM. The effect of concentration, frequency of application and runoff of Si sprays applied to the foliage was evaluated for control of PM of zucchini. Silicon (250-1000 mg .-1) + BK (0.25 m. .-1), was sprayed onto zucchini plants at frequencies of 1-3 wk-1. Spraying Si reduced the severity of PM significantly (P < 0.05). Regardless of the concentration of Si, the best results were obtained when the frequency of the treatment was increased, and when spray drift or spray runoff were allowed to reach the rhizosphere of the plants. When Si was applied onto leaves, direct contact between the spray and the pathogen resulted in mycelial death. Part of the spray (i.e., drift and runoff) was absorbed by plant roots, and subsequently played an important role in the health of the plants. If affordable, soluble Si should be included in nutrient solutions of hydroponics or supplied with overhead irrigation schemes when PM susceptible crops are grown. Under greenhouse conditions, application of BCAs, with or without Si, reduced the severity and development of PM significantly (P < 0.001). Application of Si significantly reduced the severity and AUDPC values of PM (P < 0.05 for both parameters). Silicon alone reduced the final disease level and AUDPC values of PM by 23-32%, and improved the efficacy of most BCAs. In the course of the investigation, antagonistic fungi consistently provided superior performances to bacterial isolates, providing disease control levels of up to 90%. Higher overall disease levels reduced the efficacy of Si against PM, but did not affect the efficacy of BCAs. Under field conditions, Si alone reduced disease by 32-70%, Isolate B15 reduced disease by 30-53% and Isolate B15 + Si reduced disease by 33-65%. Other BCAs applied alone or together with Si reduced the disease level by 9-68%. Most BCAs reduced AUDPC values of PM significantly. For most antagonists, better efficacy was obtained when Si was drenched into the rhizosphere of the plant. However, efficacy of some of the BCAs and Si were affected by environmental conditions in the field. Repeated trials and better understanding of how to use Si and the BCAs, in terms of their concentration and application frequency, and their interactions with the plant and the environment, are needed before they can be used for the commercial control of PM. Elemental analysis was conducted to determine the impact of differing application levels of silicon (Si) in a form of potassium silicate (KSi) in solution in terms of Si accumulation and selected elements in different tissues of zucchini and zinnia and growth of these plants, and to study the effect of PM on the levels of selected elements in these two plant species. Plants were grown in re-circulating nutrient solutions supplied with Si at different concentrations and elemental composition in different parts were analysed using EDX and ICP-OES. Increased levels of Si in the solution increased the levels of Si in leaves and roots of both plants without affecting its distribution to other plant parts. In zucchini, the roots accumulated the highest levels of Si, substantially more than in the shoots. In contrast with zinnia, accumulation of Si was highest in the leaves. Accumulation of potassium (K) in shoots of both plants increased with increased levels of KSi in the nutrient solution. However, K levels in flower of zinnia, fruits of zucchini and roots of both plants remained unaffected. Increased level of Si reduced accumulation of calcium (Ca) in both plants. Adding Si into the nutrient solution at 50 mg .-1 resulted in increased growth of zucchini and increased uptake of P, Ca, and Mg by both plant species. However, application of higher levels of Si did not result in any further biomass increase in zucchini. Levels of Si in the nutrient solution had no effects on elemental composition and characteristics of the fruits of zucchini. In both plant species, the presence of PM on the leaves of plants resulted in these leaves accumulating higher levels of Si and Ca, but less P, than leaves of uninfected plants exposed to the same levels of soluble Si. The highest concentrations of Si were observed in leaf areas infected with PM, and around the bases of trichomes. For optimum disease control and maximum accumulation of different elements in these two plants, hydroponic applications of Si at 50-150 mg .-1 is recommended. Five selected biocontrol agents and potassium silicate, used as source of soluble Si, were tested under hydroponic conditions at various concentrations against PM of zinnia (Glovinomyces cichoracearum (DC) Gelyuta, V.P.). Application of BCAs resulted in reductions in final disease level and AUDPC values of PM by 38-68% and 30-65%, respectively. Both severity and AUDPC values of PM were reduced by 87-95% when plants were supplied with Si (50-200 mg .-1). It is proposed that the provision of a continuous supply of Si and the ability of this plant species to accumulate high levels of Si in its leaves were the major reasons for the good response of zinnia to Si treatments against PM. Silicon played a protective role before infection and suppressed development of PM after infection. The combination of the best selected BCAs and Si can be used as an effective control option against PM of zinnia when grown in hydroponic system.