Efficacy of Bordeaux mixture in reducing pecan scab in the southeastern USA

Pecan scab (Venturia effusa) is the most important yield-limiting disease of pecan (Carya illinoinensis) in the southeastern USA. On susceptible cultivars, which are widely grown, conventional fungicides are applied intensively to minimize losses to the disease. Producers of organic pecan must either select resistant cultivars at planting or find alternative methods to manage scab if the cultivars are susceptible. There has been little research on the efficacy of organic products against scab, although historically, Bordeaux mixture was applied (until the late 1950s). We assessed the efficacy of Bordeaux mixture on the cultivar Stuart at two locations in Georgia from 2011 to 2016. Rainfall varied among seasons and locations and impacted severity on the non-treated controls. Thus, on mature fruit at the end of the season, the most severe scab occurred at the Colquitt location (46.63 to 71.84% fruit area diseased) compared to the Byron location (2.98 to 25.27% fruit area diseased). There was a significant reduction in the severity of scab on fruit due to the Bordeaux mixture treatment at the Byron location in 2011 (F = 17.0, P < 0.0001) and 2014 (F = 4.5, P = 0.04) and at the Colquitt location in 2014 (F = 30.3, P < 0.0001) only. In other seasons, there was no significant effect at either location. In most seasons and assessments, the severity of scab was associated with reduced fruit weight. The results are discussed in relation to tree height and fungicide coverage, the cultivar, and weather conditions. Based on the applied spray schedule, the results indicate Bordeaux mixture has limited effect in reducing pecan scab on cv Stuart, and was somewhat unreliable between seasons. We believe further testing of Bordeaux mixture is warranted on other cultivars, and efficacy should be compared to that of other organic fungicides. If considering planting a new pecan orchard to be managed organically, growers in the southeastern USA should consider selection of scab-resistant cultivars as an option.