Emergence of boscalid-resistant strains of Erysiphe necator in French vineyards

The grapevine powdery mildew Erysiphe necator (E. necator) is an obligate pathogen. Powdery mildew-diseased vines show an important reduction in plant size, winter hardiness and grape yield. Even a low-level infection with powdery mildew was shown to taint wine and ultimately reduce wine quality. For many years, succinate dehydrogenase inhibitor (SDHI) fungicides, mainly the new generation active ingredients (AIs) boscalid, penthiopyrad and fluopyram, have been widely used to control powdery mildew in grapevines. The repeated use of fungicides (mainly boscalid) has resulted in the emergence of resistant microorganisms such as Botrytis cinerea (B. cinerea). However, boscalid resistance was never observed in E. necator. In this study, a large-scale survey of French grapevine field populations of E. necator revealed many field populations with low sensitivity to boscalid. Single spore strains originating from collected resistant populations showed Half maximal effective concentration (EC50) values greater than 100 mg L−1, and strains originating from boscalid sensitive populations showed EC50 values lower than 1 mg L−1. The complete nucleotide sequences of the EnSdhB succinate dehydrogenase of sensitive and resistant single spore strains revealed that H242R and H242Y substitutions in the EnSdhB succinate dehydrogenase subunit conferred E. necator resistance to boscalid. No cross-resistance of E. necator strains bearing H242R and H242Y substitutions in EnSdhB succinate dehydrogenase to fluxapyroxad and fluopyram was noticed. Therefore, our results highlight the emergence of resistance to boscalid activity in French vineyards and warrant the need of the implementation of risk assessment strategies to maintain effective grapevine protection against powdery mildew.