Sensitivity of Venturia inaequalis to fl uxapyroxad (Serkadis) in industrial plantations in the South of Russia

 Th e apple scab pathogen <i>Venturia inaequalis</i> is claimed to be the most commonly encountered pathogen of apple orchards in temperate climates. Traditional orchard protection based on repeated application of fungicides remains relevant. Widespread use of fungicides with narrowly focused action leads to a gradual selection of resistant pathogen biotypes, an increase in their share in the population and, subsequently, to a loss of eff ectiveness of chemicals. Monitoring the pathogen sensitivity to fungicides, including new ones that have appeared on the pesticide market relatively recently, such as succinate dehydrogenase inhibitors (SDHI), constitutes an important aspect in the development of an anti-resistance orchard protection program. Th e present paper evaluates the sensitivity of the fungus to fl uxapyroxad using in vitro discriminatory concentration based on relative growth (RG) of mycelium on solid medium. Monospore isolates of the pathogen were grown on potato glucose agar with and without 0.25 μg/mL of fl uxapyroxad. Th e study involved a total of 106 isolates of the pathogen collected from two industrial appl e orchards. Th e mean RG values of these populations differed signifi cantly. Th e orchards with a history of active ingredient application demonstrated the presence of isolates with RG of mycelium above 70 and 80%, thereby indicating the development of resistance in them. In one of the orchards, the proportion of isolates at a discriminatory threshold of RG > 80 % was twice as high as in the other one. SDHI fungicides and in particular fl uxapyroxad refer to narrowly acting fungicides, which makes <i>Venturia inaequalis</i> highly probable to develop resistance to them. Th e present study is apparently the fi rst in Russia to demonstrate the presence of pathogen forms wit h reduced sensitivity to this active ingredient. Compliance with anti-resistant FRAC recommendations when using single-site fungicides and constant monitoring of the pathogen sensitivity to them form an important condition for their long-term use.

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