Soil streptomycetes with in vitro activity against the yam pathogens curvularia eragrostides and colletotrichum gloeosporioides

Yam (Dioscorea cayennensis Lam), an important crop in the Northeastern region of Brazil, is susceptible to the attack of several phytopathogenic fungi, such as Curvularia eragrostides (Henn.) Meyer and Colletotrichum gloeosporioides (Penz.), which cause leaf spot diseases. This work aimed to evaluate the effect of six actinomycete isolates (S. thermotolerans, S. griseus subsp. griseus, Streptomyces sp. N0035, S. purpurascens, and two isolates identified as Streptomyces sp.), as well as the effect of their secondary metabolites, on mycelium growth and spore germination of C. eragrostides and C. gloeosporioides, from yam plants, cultivar da Costa. Additionally, the chitinolytic activity of these actinomycete strains was determined in a minimum salt growth medium with chitin as the sole carbon source. There was significant interaction between the actinomycete isolates and the phytopathogenic fungi for spore germination and mycelium growth inhibition. Streptomyces sp. codified as AC 26 was more efficient in inhibiting spore germination of C. eragrostides, while S. thermotolerans and Streptomyces sp. N0035 were more efficient for C. gloeosporioides. Streptomyces sp. (AC 26) was the most efficient in inhibiting mycelium growth of both fungi, and the inhibitory effect was positively correlated to metabolite concentration. S. griseus subsp. griseus did not show chitinolytic activity and did not control mycelium growth of both fungi. The actinomycete strains evaluated in this study can be considered potential biological agents for controlling yam phytopathogenic fungi associated with leaf spot diseases.