Evaluation of a phosphite fungicide to control pecan scab in the southeastern USA

Abstract

The efficacy of phosphite, a potential elicitor of systemically acquired resistance (SAR) was compared to the protectant fungicide triphenyltin hydroxide (TPTH) to control pecan scab caused by Fusicladium effusum. Efficacy was evaluated in four field experiments over a two-year period involving biweekly foliar applications of both fungicides to trees of five susceptible cultivars of pecan (Carya illinoinensis) and assessment of disease severity on foliage and fruit. Both phosphite and TPTH reduced scab severity on foliage equally well compared to the non-treated control, with the exception of one of the TPTH treatments in 2010. Both phosphite and TPTH provided equally good control of disease early in fruit development (Jul/Aug). However, by the final assessment (Sep/Oct), scab severity on phosphite-treated trees was most often greater than those receiving TPTH and in 2010, severity was equivalent to the non-treated control. Despite a suggested lack of late-season protection with phosphite, there was no difference in fruit volume between phosphite and TPTH-treated plots in 2009, and no difference in nut volume in 2010, although there were treatment differences in kernel weight and fruit weight in 2010. Phosphite-treated trees showed some symptoms of phytotoxicity. Regression analysis demonstrated the effect of scab on yield loss and confirmed the value of scab control on susceptible cultivars. In-vitro tests showed that phosphite is toxic to scab at rates applied in the field, thus implying direct fungitoxicity. Results indicate that phosphite provides useful control of pecan scab on both foliage and fruit early in the growing season, but might not provide prolonged late-season protection compared to an industry standard (i.e., TPTH).