Disease Distribution and Fungicide Coverage in Pecan Trees

Pecan scab (Fusicladium effusum G. Winter) is the major disease of pecan (Carya ilinoinensis) in the southeastern US, and severe epidemics on susceptible cultivars can result in complete yield loss. Anecdotal evidence indicates adequate scab control in the tops of tall trees in mature orchards is difficult, but no data exist demonstrating the levels of disease control achieved at different heights in the canopy when using conventional, ground-based air-blast sprayers. Experiments on 16-m-tall trees of cultivars 'Desirable' (2010, 2011) and 'Wichita' (2011) compared severity of scab at 5 heights (<5.0, 5.0-7.5, 7.5-10.0, 10.0-12.5 and >12.5 m above ground) on fungicide-treated and non-treated trees. There was a consistent, significant (P<0.0001) difference in severity of scab on fruit of treated and non-treated trees to a height of 10 m in early-August, which was also true in October (P<0.0001-0.04), six weeks after the last fungicide application. In additional experiments, spray coverage was studied using water-sensitive cards placed in the inner and outer canopy of mature pecan trees at 0.0, 5.0, 7.5, 10.0, 12.0 and 15.0 m above ground. Spray coverage was measured, using digital image analysis, as the percentage card area covered. There were spray coverage differences between the inner and outer canopy (P=0.05). Spray coverage was uniform up to 10 m, but at heights >10 m significantly less (P=0.0001) surface area was covered by spray. Thus, in trees receiving fungicide from an air-blast sprayer, disease is reduced in the lower canopy but there is little effect in the upper canopy. In fact the vertical distribution of spray reflected the height to which the severity of pecan scab was reduced. Trees in pecan orchards are often >10-15 m tall and older trees can reach >30 m, so to achieve adequate scab control additional aerial application of fungicide may be required.