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Accueil du site → Doctorat → Allemagne → 2008 → Biological control of Striga hermonthica (Del.) Benth. using formulated mycoherbicides under Sudan field conditions

Universität Hohenheim (2008)

Biological control of Striga hermonthica (Del.) Benth. using formulated mycoherbicides under Sudan field conditions

Zahran, Eldur

Titre : Biological control of Striga hermonthica (Del.) Benth. using formulated mycoherbicides under Sudan field conditions

Die biologische Kontrolle von Striga hermonthica (Del.) Benth. mit formulierten Mycoherbiziden unter Feldbedingungen im Sudan

Auteur : Zahran, Eldur

Université de soutenance : Universität Hohenheim

Grade : Doctor der Agrarwissenschaften ( / Ph.D. in Agricultural Sciences) 2008

Résumé partiel
Striga hermonthica is a parasitic flowering plant belonging to the family Orobanchaceae. It is a root parasite that attacks sorghum, maize, millet and several grass weeds in the semi-arid Tropics. In Sudan, Striga is widespread in irrigated and rainfed areas and considered the main biotic constraint in production of sorghum, the main staple food for the majority of Sudanese people. More than 500,000 hectares under rainfed cultivation are heavily infested with Striga, which commonly results in significant yield losses of 70 ? 100%. It has become obvious that there is no simple, fast and inexpensive solution to the Striga problem in Africa. Biological control is considered a potential cost-effective and environmentally safe means for reducing weed populations in crops, forests, or rangelands where low profit margins prevent large herbicide expenditure. Biological control using microorganisms (especially phytopathogenic fungi) showed a high efficacy in controlling S. hermonthica under controlled and field conditions. However, so far it did not come to practical field application. This could be attributed to environmental obstacles or due to the lack of appropriate delivery systems. The pathogenicity of two fungal isolates indigenous to Sudan (Fusarium nygamai [FN] and F. ?Abuharaz ? [FA] isolate) against Striga has been studied using infected sorghum grains or a spore suspension as inoculum. These formulations were very effective in controlling Striga under controlled and natural conditions ; however, a high level of fungal inoculum (approximately 800 kg ha-1 for the grain inoculum) would be required for effective control, which arises a lot of problems e.g. concerning sterilization and transportation. Such problems can be overcome by adopting an appropriate formulation technology. Granular formulations such as ?Pesta ? and alginate pellets were found to be suitable delivery systems for controlling weeds. ?Pesta ? granules are made by encapsulating bioagents in a gluten matrix. Alginate formulations are prepared by incorporating the biocontrol agent ?s propagules in a sodium alginate solution, which is dripped to a calcium chloride or calcium gluconate solution. Alginate pellets are then formed by ionotrophic gelation. The main objectives of this study were : (a) to study the efficacy of the two Fusarium species in controlling Striga under field conditions using ?Pesta ? and alginate formulations, (b) evaluate the effect on sorghum yield, (c) determine the optimum dose of the formulated material, (d) investigate the persistence of the formulated fungal isolates in the soil, and (e) study the efficacy of seed treatments as an alternative delivery system. Furthermore, for environmental safety reasons the newly isolated F. ?Abuharaz ? isolate was tested for its ability to produce some of the most important mycotoxins. Harvested sorghum seeds out of the fungus-treated plots were also investigated for their mycotoxins content. A prerequisite to be able to formulate biocontrol fungi is the development of an inexpensive method of inoculum production that yields sufficient biomass containing viable, highly virulent propagules. Chlamydospores are the soil-persisting propagules of many Fusarium species and considered as ideal propagules to be used in granular formulations. For this reason, finding a medium suitable for the production of chlamydospores by the two Fusarium isolates was one of the specific objectives of this study. Different media were tested among them Special Nutrient-poor Broth (SNB) + yeast gave the highest number of chlamydospores (105 ml-1) in both isolates throughout the incubation period. However, both isolates generally did not form sufficient chlamydospores to be used within a bioherbicide formulation. Richard ?s solution gave the highest number of microconidia (108 ml-1) after five days of incubation and hence it was selected as growth medium for formulation purposes throughout this study.


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