Diversity of Sclerotium rolfsii antagonist fungi isolated from soils of the rhizosphere of tomato crops and identification of some antifungal compounds.

Sclerotium rolfsii Sacc. the causative agent of white rot is one of the destructive pathogens of nightshade crops. In Côte d'Ivoire, this fungal pathogen constitutes a major constraint for the cultivation of tomato (Solanum lycopersicum) with 41.01% crop losses in humid forest areas. Controlling this fungus with synthetic chemicals can be effective, but harmful to human health and the environment. The use of biological control agents could be an alternative approach to control S. rolfsii. In this perspective, the objective of this work was to select fungi from the rhizosphere of tomato crops capable of inhibiting the growth of S. rolfsii. To do this, 153 fungi were isolated from the rhizosphere and from direct confrontation tests 10 fungi whose antagonistic power of S. rolfsii varied between 27 and 60% were selected. Molecular identification (ITS) of these antagonist fungi revealed that the isolates belonged to the genera Talaromyces sp. (n = 4), Trichoderma sp. (n = 3), Penicillium sp. (n = 2) and Clonostachys sp. (n = 1). Among these fungi, Talaromyces purpureogenus and Talaromyces assiutensis were able to diffuse compounds in agar capable of inhibiting the growth of S. rolfsii. The chemical study of these 2 fungi made it possible to identify mitorubrin and mitorubrinol produced by T. purpureogenus and spiculisporic acid produced by T. assiutensis. Mitorubrin and mitorubrinol had inhibitory activities of 100 and 70% at 10 mg/mL, respectively, whereas spiculisporic acid showed moderate inhibition of 38 at 20 mg/mL of the growth of S. rolfsii; however, its abundant production by the fungus could be an advantage in the control of this phytopathogen. Isolated from the same biotope as S. rolfsii, T. purpureogenus and T. assiutensis represent favorable candidates for the biological control against S. rolfsii.

Bioconversion of antifungal viridin to phytotoxin viridiol by environmental non-viridin producing microorganisms.

Biotransformation of viridin, an antifungal produced by biocontrol agent, with non-viridin producing microorganisms is studied. The results show that some environmental non-targeted microorganisms are able to reduce it in the known phytotoxin viridiol, and its 3-epimer. Consequently, this reduction, which happens in some cases by detoxification mechanism, could be disastrous for the plant in a biocontrol of plant disease. However, a process fermentation/biotransformation could be an efficient approach for the preparation of this phytotoxin.

Inhibition of Phytophthora species, agents of cocoa black pod disease, by secondary metabolites of Trichoderma species.

Cocoa production is affected by the black pod disease caused by several Phytophthora species that bring, about each year, an estimated loss of 44% of world production. Chemical control remains expensive and poses an enormous risk of poisoning for the users and the environment. Biocontrol by using antagonistic microorganisms has become an alternative to the integrated control strategy against this disease. Trichoderma viride T7, T. harzanium T40, and T. asperellum T54, which showed in vivo and in vitro antagonistic activity against P. palmivora, were cultured and mycelia extracted. Inhibition activity of crude extracts was determined, and then organic compounds were isolated and characterized. The in vitro effect of each compound on the conidia germination and mycelia growth of four P. palmivora, two P. megakaria, and one P. capsici was evaluated. T. viride that displayed best activities produced two active metabolites, viridin and gliovirin, against P. palmivora and P. megakaria strains. However, no activity against P. capsici was observed. Besides being active separately, these two compounds have a synergistic effect for both inhibitions, mycelia growth and conidia germination. These results provide the basis for the development of a low-impact pesticide based on a mixture of viridin and gliovirine.