Fruit and nut weight in pecan tree canopies in relation to the severity of pecan scab at different heights

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SummaryFusicladium effusum is the cause of pecan scab, the most destructive disease of pecan in the south‐eastern USA. The aim of the study was to determine how scab severity at different sample heights in tall trees (14–16 m) affected fruit and nut weight at those same heights. Three experiments were performed in 2010‐2011 on trees of scab‐susceptible cultivars (cvs. Desirable and Wichita) and either received fungicide using an air‐blast sprayer or were non‐treated controls. Samples for measures of fruit and nut weight were taken at 0–5.0, 5.0+–7.5, 7.5+–10.0, 10.0+–12.5 and 12.5+–15.0 m (and were assessed for scab severity). A general linear mixed model analysis showed main effects of sample height, fungicide treatment and sample height × fungicide treatment interactions in all experiments on fruit and nut weight and on disease severity. Control trees had significantly more severe disease on fruit lower in the canopy, while fungicide‐treated trees either had similar disease severity at all heights or slightly more severe disease in the upper canopy. On control trees, where there was a linear relationships between fruit or nut weight and sample height, it was invariably positive (R2= .85–.94). On fungicide‐treated trees, fruit or nut weight most often had a negative linear relationship with sample height (R2=.69–.89). When significant, in control and treated trees, there was invariably a negative linear relationship between fruit or nut weight and scab severity (R2=.66–.94). Gradients in fungicide coverage and scab severity impact fruit and nut weights that can have ramifications on yield for a tree as a whole. Understanding the distribution of scab in pecan and other tall tree canopies provides a basis for developing management strategies and fungicide application methods to uniformly minimize disease.