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Efficacy of Bordeaux mixture to control pecan scab in large-plot experiments

Michael Hotchkiss: USDA ARS

<div><em>Venturia effusa</em> causes scab, the most important disease on pecan in the southeastern USA. Organic fungicides have not been widely tested for efficacy against scab on susceptible cultivars. A large-plot experiment was used to test the efficacy of the traditionally-used fungicide against scab, Bordeaux mixture (BM), on trees of cv. Stuart at two sites (Colquitt and Byron) in GA. The experiments were conducted in 2011, 2012, 2014, 2015 and 2016 (Colquitt only in 2014 and 2015). BM was applied 6 times using an orchard sprayer as follows: 4 lb CuSO<sub>4</sub>:1 lb hydrated lime, for the 1<sup>st</sup> spray; followed by 6 lb CuSO<sub>4</sub>:2 lb hydrated lime for the 2<sup>nd</sup> - 6<sup>th</sup> sprays applied in 100 gallons per acre. Fruit samples were assessed for scab severity twice. At the time of the 1<sup>st</sup> assessment, only in 2011 did BM reduce scab severity (F=56.4, P<0.0001; control = 4.14% fruit area diseased, BM = 1.08% area diseased (a 73.9% reduction). BM significantly reduced severity of scab at the time of the 2<sup>nd</sup> assessment at Byron in 2011 (F=12.3, P=0.0005) and 2014 (F=4.4, P=0.04), and at the Colquitt location in 2014 (F=30.0, P<0.0001) only. The percentage reduction in severity of scab was 43.9%, 16.2% and 24.4%, respectively. In 2015 at the Colquitt location there was an increase (16.3%) in the severity of scab on fruit of BM-treated trees (F=8.8, P=0.003). The limited control of scab by BM indicates resistant cultivars are preferable in organic production systems in scab-prone areas.</div>