Occurrence and dynamics of resistance to a QoI and a DMI fungicide in Erysiphe necator the causal agent of grape powdery mildew in Israel

Abstract

Powdery mildew incited by the fungus Erysiphe necator is a destructive disease of grapes in Israel and elsewhere, attacking green parts of the vine and reducing yield and wine quality. Demethylation inhibitors (DMI) and quinone outside inhibitors (QoI) fungicides are frequently used to control the disease. However, starting in 2005 and 2007, respectively, we observed reduced control efficacy of DMI and QoI fungicides in Israel. The resistance of E. necator to DMI or QoI has not been phenotypically described or genetically characterized in the Middle Eastern region, and the seasonal dynamics of resistance have been under-studied worldwide. Field experiments performed during 2017 and 2018 in the Judean-foothills and Upper Galilee regions, showed reduced efficacy of both fungicides. Sensitivity of E. necator to the fungicides was determined in grape leaf disks bioassays. Isolates collected from sprayed plots exhibited EC50 values of 100–1000 and 2–8 μg/mL for QoI and DMI, respectively, while isolates collected from non-sprayed vines showed EC50 values of 0.5–1.6 and 0.04–1.8 μg/mL, respectively. Resistant isolates carried the genetic mutations to DMI (Y136F) and QoI (G143A). We conclude that the recently reduced control efficacy of the disease in Israeli vineyards resulted from fungicide resistance. Seasonal dynamics of DMI and QoI mutant isolates in the vineyards were different. The proportion of QoI mutant isolates increased and became dominant by mid and late season, while DMI mutant isolates dominated mid-season, and DMI-wild-type isolates dominated at the beginning and end of the season when selection pressure was low.