atroviride and Phomopsis sp., and the other in which R. solani growth is
weakly inhibited (A. longipes, E. nigrum). In this study, T. atroviride and Phomopsis sp. were found to be the best antagonists against R. solani. Confocal microscopy observations of all the fungal BCAs used in this study confirmed that they act differently against R. solani. The active antagonists limit themselves to the pathogens and block their development by winding around the hyphae. However, T. atroviride showed evidence of penetration into pathogen hyphae. This mechanism has been reported (Benhamou & Chet, 1996) using electron microscopy. Whipps (2001) showed that Trichoderma spp. includes LDK378 cell line several species that produce antibiotics against different plant pathogens and, indeed, many were studied and some have been used as commercial BCAs. Whipps (2001) also mentioned that competition for nutrients and space is Z-VAD-FMK ic50 another possible mechanism by which BCAs suppress or reduce pathogen infections. For example, T. atroviride can parasitize many soilborne pathogens, such as R. solani, Sclerotium rolfstii, Fusarium sp., Phytophthora sp., and Pythium sp. Trichoderma has been reported to form specialized structures upon contact with its target, in particular, the mycoparasite coils around the host hyphae (Herrera-Estrella & Chet, 1999). There are several studies showing the implication of the genes encoding hydrolytic enzymes and the
secretion of these enzymes in the mycoparasitism interactions (Kim et al., 2002). On the other hand, E. nigrum limits pathogen development by growing along R. solani hyphae and inducing their lysis. Epicoccum nigrum, also known in the literature as Epicoccum purpurascens Ehrenb, ex Schlecht., is an anamorphic fungus that produces darkly pigmented (Fig. 1e) muriform conidia on short conidiophores borne on the surface of a sporodochium, a superficial, cushion-like mass of pseudoparenchyma-like hyphal cells. It has been used as a BCA for certain fungal diseases of plants, apple brown rot (Monilia laxa) and damping-off (Hashem & Ali, 2004). However, its efficacy has never been evaluated
against Rhizoctonia diseases. Consequently, our work is the first investigation showing the role of this fungus in controlling R. solani diseases on potato. The results obtained for the production of volatile substances showed that all antagonist Rho isolates produce volatile substances acting against this pathogenic fungus. However, the inhibition of radial pathogenic fungus growth remains inferior to that observed in the dual culture assay. It has been shown that Trichoderma species are highly effective BCAs of soilborne plant pathogens and can produce volatile and nonvolatile antibiotics that inhibit the growth of other pathogens (R. solani, Heterobasidium annosum, and Fusarium oxysporum) (Haran et al., 1996). Our work is the first investigation to test both fungal genera Phomopsis and Alternaria for a role in controlling R. solani diseases.