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POSTERS: Fungicide and antibiotic resistance

Assessing the impact of the G137S substitution in Venturia effusa on QoI efficacy in commercial pecan orchards
Timothy Brenneman - University of Georgia. Jeffrey Standish- University of Georgia, Katherine Stevenson- University of Georgia, Kory Herrington- University of Georgia

Every year Georgia pecan growers lose millions of dollars to pecan scab, caused by Venturia effusa. Scab management relies heavily on fungicides, including the quinone outside inhbitors (QoI; FRAC group 11). Recently, reduced QoI efficacy has been observed in a few Georgia orchards and previous research revealed that an amino acid substitution from glycine to serine at position 137 (G137S) within the V. effusa cytochrome b gene leads to partial QoI-resistance. However, the effect of G137S on QoI efficacy in the orchard is unknown. Thus, field trials were conducted in 2018 across six locations in southwest Georgia to determine the impact of G137S. At each location, individual terminals were treated biweekly with Abound (azoxystrobin) at a rate of 0.88 L/ha, for 10 applications/year. Nut scab severity was evaluated in August and approximately 20 isolates were collected from unsprayed portions of trees at each location to screen for mechanisms of resistance. Percent control of nut scab ranged from 25.7 to 75.2% in 2018 and G137S was found in three of the six orchards, ranging in incidence from 0.0 to 30.8%; no other mechanisms were present. Percent control decreased linearly with increasing G137S frequency, indicating that the reduced QoI efficacy may have been a result of a greater number of G137S isolates in the population. Monitoring Georgia pecan orchards for isolates harboring G137S could allow for the detection of major shifts in efficacy before they occur.