Efficacy of a Beauveria bassiana-Based Biofungicide Against Strawberry Powdery Mildew Caused by Podosphaera aphanis Under Various Conditions and Its Potential Mode of Action

Powdery mildew, caused by Podosphaera aphanis, is a severe disease affecting strawberry production in Japan. The biofungicide BotaniGard ES, whose active ingredient is Beauveria bassiana strain GHA, was reported to be effective against P. aphanis. In the present study, the efficacy of BotaniGard was clarified under a range of conditions to evaluate its practicality in the greenhouse. In greenhouse efficacy trials, the control efficacy of BotaniGard was relatively high (70.4%–100%) against the disease under low disease pressure, but low (28.1%–53.4%) under high disease pressure. In addition, the application 6 h before inoculation was the most effective compared with application 7 days before or 7 days after inoculation. The application of BotaniGard at 1-week intervals (control efficacy of 66.5%) was more effective against powdery mildew on leaves than at 2- or 3-week intervals. In contrast, the application of BotaniGard was not effective against powdery mildew on fruits, regardless of application interval, although most of the chemical fungicides tested for comparison were effective. RNA-seq analysis indicated that the application of BotaniGard upregulated the defence response, systemic acquired resistance and chitinase activity in leaves. These results indicate that BotaniGard is effective in controlling strawberry powdery mildew on leaves under low disease pressure, but not on fruits, and the mode of action might be the activation of induced plant defences. This study encourages a detailed evaluation of the efficacy of biofungicides to promote their appropriate use in greenhouse production.