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Dynamics of fungicide sensitivity in Venturia effusa and fungicide efficacy under field conditions.

Jeffrey Standish: University of Georgia

<div><em>Venturia effusa, </em>which causes scab, the most damaging disease of pecan, has developed resistance to fungicides that were once very effective. A 2-year study was conducted to explore the relationship between laboratory-based sensitivity to fentin hydroxide (TPTH) and tebuconazole (TEB) in <em>V. effusa </em>and the efficacy of these products in managing scab under field conditions. Scab epidemics were monitored on trees each receiving ten applications of TPTH (Super Tin 4L), TEB (Orius 3.6F), azoxystrobin (AZ; Abound), AZ + TEB, TPTH + TEB, or no fungicide (NTC). Sensitivity of <em>V. effusa</em> on leaflets from treated trees was assessed in June and September of both years. In 2016, mean relative germination (RGe) on 30 µg/ml TPTH was 11% and 41% (June and Sept., respectively), and in 2017, RGe was 4% and 1%. Mean relative growth (RGr) on 1 µg/ml TEB in 2016 was 46% and 35%, and 69% and 56% in 2017. Leaf and nut scab intensity were significantly lower in both years on trees treated with AZ, AZ + TEB, or TPTH + TEB when compared to NTC and TEB-treated trees. Compared to the NTC, TEB did not significantly reduce leaf scab in 2017 or nut scab in either year, indicating that an RGr value between 46% and 69% is likely to result in a control failure on TEB-treated trees. Although better control was expected, TPTH reduced scab with RGe values between 1% and 41%. These results will be valuable in developing critical fungicide sensitivity thresholds to better predict fungicide performance.</div>