Scab Intensity in Pecan Trees in Relation to Hedge-Pruning Methods.

Abstract

Pecan is a valuable nut crop cultivated in the southeastern United States. Among the major yield-limiting factors in the region is scab, caused by the plant pathogenic fungus Venturia effusa. Managing scab in tall trees (15 to 25+ m) in pecan orchards is challenging because of the limitations of getting sufficient spray coverage throughout the canopy. We explored the effects of hedge pruning on scab in three orchards: 14-m-tall cv. Desirable trees winter hedge pruned on alternate sides to 11 m (site 1), 18-m-tall cv. Stuart trees hedge pruned on both sides simultaneously to 11 m (site 2), and 15-m-tall cv. Caddo trees winter hedge pruned in winter versus summer to 11 m (site 3). At site 1 and 2, hedge-pruned trees were compared with nonpruned control trees. All trees received recommended fungicide applications to control scab via an air-blast sprayer. Disease incidence and/or severity was assessed at different sample heights on shoots, foliage, and fruit during three seasons (2020, 2021, and 2022). At site 1 the hedge-pruned trees often had significantly or numerically more severe scab on foliage and fruit compared with the control trees, although the differences were mostly small. The frequency of mature fruit with scab severity <10% was greatest on control trees in 2021 and 2022. At site 2, there were few differences between hedge-pruned and control trees (on fruit, scab severity was either significantly less on hedge-pruned trees or not different from the control), but the frequency of mature fruit with scab severity <10% was consistently greatest on hedge-pruned trees. At site 3, scab intensity was low, and there were no significant differences in scab severity between winter- and summer-pruning treatments. At sites 1 and 2, there was generally more severe scab at greater sample heights compared with low in the canopy. At site 3 there was little effect of height on disease. The benefit of hedge pruning likely increases with tree height in scab-susceptible cultivars. If a tree is taller than ∼15 m, a greater proportion of the fruit will be within reach of efficacious spray coverage from air-blast sprayers.