Cross-resistance studies of metrafenone resistant isolates of Erysiphe necator

Erysiphe necator is one of the major fungal diseases of cultivated grapevine worldwide and it is managed almost exclusively by fungicides. It has already developed resistance to several fungicide groups including quinone outside inhibitors (QoI) and sterol biosynthesis inhibitors (SBI). Metrafenone is a fungicide belonging to benzophenones (FRAC group U8), and it is used specifically to control powdery mildews. Although its exact mode of action is not known, it is different from other fungicides used in powdery mildew management and is therefore a valuable choice in fungicide rotation programs. Recently, we have described metrafenone resistance in a population of E. necator in northern Italy. Thirteen strains were obtained from Franciacorta area and analyzed for their sensitivity to metrafenone in terms of mycelium growth and sporulation. All isolates grew abundantly on control plants; after 14 days of growth their average colony area was 89.4 mm2 (standard deviation, SD 39.9) and they produced on average 2212.1 spores/cm2 (SD. 1668.8). Two strains were sensitive, and they did not grow at the metrafenone concentration used in field (125 mg/l a.i.). The resistant strains grew and sporulated similarly to control at metrafenone field concentration, and even at metrafenone concentration of 1250 mg/l. Moreover, we studied the cross-resistance of E. necator metrafenone-resistant strains to pyriofenone, which belongs to the same FRAC group and two fungicides representative of QoI and SBI groups, azoxystrobin and myclobutanil. The leaf surface of pyriofenone-treated plants was 100% colonized by metrafenone-resistant strains, confirming cross-resistance between metrafenone and pyriofenone, whereas they were fully inhibited by azoxystrobin and myclobutanil, indicating absence of cross-resistance with QoI and SBI.