Occurrence of fungicide resistant Venturia effusa in Georgia pecan orchards and implications for disease management

Georgia pecan growers spend millions of dollars annually on fungicides to manage pecan scab caused by Venturia effusa. Shifts in V. effusa sensitivity to major fungicide classes have been observed with reports of control failures, but these were never linked directly to shifts in sensitivity. In 2017 and 2018, pecan orchards in Georgia were surveyed for reduced sensitivity and efficacy of fentin hydroxide (TPTH) and azoxystrobin. The G137S substitution of the cytochrome b gene was identified in 39 of 196 total isolates. There was a significant negative linear relationship between both RGe on TPTH and frequency of the G137S mutation and the relative control of scab with each fungicide. This is the first report of the G137S substitution impacting QoI efficacy on pecan scab in the field. TPTH is still generally effective on scab, and there is an apparent fitness cost associated with reduced sensitivity to TPTH.