Evidence of soil‐located competition as the cause of the reduction of sunflower verticillium wilt by entomopathogenic fungi

Abstract

The increasing limitation of agrochemicals for disease control is a major challenge for European agriculture and a spur to developing environmentally friendly approaches such as biological control. Entomopathogenic fungi, which have been used in the control of insect pests for a long time, also have other uses, such as being antagonists of fungi, including plant pathogens. We determined the in vitro effect of three strains of Metarhizium brunneum and two of Beauveria bassiana against Verticillium dahliae and Cadophora helianthi, causal agents of sunflower wilts. Both M. brunneum and B. bassiana were able to inhibit the mycelial growth of the sunflower pathogens and, according to the dual culture and microscopy results, two types of antagonism were observed as being dependent on the strain: competition and/or antibiosis. Greenhouse experiments showed that, after soil treatments with entomopathogens and plant inoculation by root immersion in conidial suspensions of V. dahliae, the entomopathogens were able to efficiently persist in the soil, and three of the four strains even significantly reduced the severity of symptoms in sunflowers. Interestingly, molecular analysis showed that all the strains were able to establish themselves as endophytes in sunflowers in the absence of V. dahliae. When the plants were inoculated with V. dahliae, we detected the pathogen, but not the entomopathogen, in the sunflowers by molecular methods. The results of this work suggest that the protection conferred by M. brunneum and B. bassiana against verticillium wilt might not be plant‐located, but is probably the consequence of their competition with V. dahliae in the soil.