Resistance of erysiphe necator to metrafenone in Northern Italy

Erysiphe necator is one of the major fungal diseases of cultivated grapevine worldwide. Its management relies almost exclusively on the use of fungicides. However, it has already developed resistance to several fungicide groups, including quinone outside inhibitors and sterol biosynthesis inhibitors. Metrafenone, belonging to benzophenones (FRAC group U8), is a fungicide used to specifically control powdery mildews. It was registered in Europe in 2006 and since then it has been extensively used on grapevine, cereals and cucurbits. Although its exact mode of action is not known, it is different from other fungicides used in powdery mildew management and therefore it represents a valuable choice in fungicide rotation programs [1]. So far, resistance to metrafenone was reported in wheat powdery mildew [2]. In the presented work we investigated metrafenone resistance in the population of E. necator in northern Italy. The dynamics of grapevine powdery mildew epidemics and metrafenone efficacy were monitored in field in three consecutive years. While 2011 was not favorable for disease development, in 2012 and 2013, almost 100% of grape clusters were infected in the control plots by mid-July, with the disease severity higher than 50%. Metrafenone protected well treated plants in the first two years, where it demonstrated the protection of grape clusters especially in terms of disease severity by more than 80%. On the contrary, no difference from control plots was observed in 2013, neither on leaves nor grape clusters. Due to the low efficacy of metrafenone in field in 2013, thirteen monoconidial 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 remaining strains showed variable response to metrafenone, and four of them grew and sporulated equally to control even at the metrafenone concentration of 1250 mg/l. Moreover, two additional resistant strains were isolated from the experimental field in 2014, after metrafenone was discontinued and the vineyard changed to biological management. To our knowledge, this is the first study reporting metrafenone resistance in Erysiphe necator. Our identification of metrafenone resistant strains will be valuable for getting an insight into its mode of action. Further studies are needed to understand its implications for the use of metrafenone in the integrated disease management of grapevine powdery mildew in vineyards, and to determine the impact of resistance on changes in the pathogen population structure, fitness and spread of the resistant strains. 1 Opalski KS, Tresch S, Kogel K-H, Grossmann K, Köhle H, Hückelhoven R. Metrafenone: studies on the mode of action of a novel cereal powdery mildew fungicide. Pest Manag Sci 2006; 62: 393–401. 2 Felsenstein F, Semar M, Stammler G. Sensitivity of wheat powdery mildew (Blumeria graminis f.sp. tritici) towards metrafenone. Gesunde Pflanz 2010; 62: 29–33.