Metarhizium brunneum and Beauveria bassiana release secondary metabolites with antagonistic activity against Verticillium dahliae and Phytophthora megasperma olive pathogens

The antifungal activity of the entomopathogenic fungi isolates Metarhizium brunneum EAMb 09/01-Su and Beauveria bassiana EABb 09/16-Su against the olive pathogens Verticillium dahliae and Phytophthora megasperma was evaluated. The crude extract and its partial purified fractions obtained from both isolates showed complete inhibition of the mycelial growth of V. dahliae. However, the same fractions obtained from B. bassiana EABb 09/16-Su were less effective against the mycelial growth of P. megasperma. Our results showed that M. brunneum EAMb 09/01-Su had the highest fungicidal activity, inhibiting the mycelial growth of both pathogens. Both fractions IV and VI from M. brunneum EAMb 09/01-Su were evaluated by the progress of the Verticillium wilt of the plant and showed clear antagonism against V. dahliae, reducing the severity of the symptoms between 38.9 and 58.9%. The active metabolite obtained from M. brunneum not only reduced the germination of V. dahliae microsclerotia but also inhibited hyphal formation, decreased the number of reproductive structures, and changed the conformation of the microsclerotial colony, preventing the formation of new microsclerotia. The evaluation of temperature, UV light and pH on the antifungal activity of the dialyzed fraction from M. brunneum showed that the antifungal activity was stable at 60 °C (2 h) and 120 °C (20 min) and to UV light radiation. However, the exposition to the dialyzed fraction at pH 7.5 and 8.5 affected the antifungal activity. The results of this study clearly showed that the entomopathogenic fungus M. brunneum EAMb 09/01-Su produces antifungal compounds that reduce the number of the pathogen propagules in the soil and the severity of Verticillium wilt. Therefore, this resource should be further studied as an alternative for the integrated management of Verticillium wilt and Phytophthora root rot, which is the most severe disease in all traditional olive-growing regions worldwide, and as a reliable and sustainable strategy for a reduced reliance on pesticides.