Resistance to metrafenone of Erysiphe necator and insights into its mode of action

Metrafenone is a benzophenone fungicide specifically used to control powdery mildews. It was registered in Europe in 2006 and since then it has been extensively used on grapevine, cereals and cucurbits. Its exact molecular mode of action is still not known, however, it is different from other fungicides used in powdery mildew management as demonstrated by the absence of cross-resistance with other known chemical classes. Early studies on Blumeria graminis f.sp. hordei and Blumeria graminis f.sp. tritici suggest that it interferes with hyphal morphogenesis, polarized hyphal growth and the establishment and maintenance of cell polarity. In 2013, poor efficacy of metrafenone to control grapevine powdery mildew (Erysiphe necator) in a field trial in northern Italy was observed. Subsequent laboratory tests confirmed that the detected lack of activity was due to the emergence of resistance in E. necator population. Eighty-five percent of monoconidial isolates collected from the vineyards in the Franciacorta area (Lombardy) grew and sporulated at the field recommended concentration of the fungicide (125 mgL-1 a.i.), and to a lesser extent even at 10-times higher concentration. Resistant strains were identified also in subsequent years in Franciacorta and in Spilimbergo area (Friuli-Venezia-Giulia). Cross-resistance studies revealed that metrafenone-resistant strains were effectively controlled by strobilurins (azoxystrobin) and inhibitors of sterol biosynthesis (myclobutanil), but were resistant to pyriofenone, fungicide belonging to the same FRAC U8 group. Microscopic observations of E. necator metrafenone-sensitive strain inoculated on grapevine leaves (cv. Chardonnay), that were preventatively treated or not with metrafenone, indicate that metrafenone does not inhibit spore germination and appressoria formation, but it blocks further development, possibly interfering with fungal penetration or development of haustoria. Identification of the developmental stage at which metrafenone acts is indispensable for further molecular studies aimed at elucidating the exact biochemical pathway of the action of and resistance to metrafenone.