Disease distribution and fungicide coverage in pecan trees

Authors: Bock, Clive; Hotchkiss, Michael - Mike; Cottrell, Ted; Wood, Bruce

Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/4/2014
Publication Date: 2/15/2015
Citation: Bock, C.H., Hotchkiss, M.W., Cottrell, T.E., Wood, B.W. 2015. Disease distribution and fungicide coverage in pecan trees. Acta Horticulturae. 1070:171-182.

Interpretive Summary: Pecan scab, caused by the plant pathogenic fungus Fusicladium effusum is the major disease of pecan in the southeastern U.S., and severe epidemics can result in complete yield loss in susceptible cultivars. Achieving adequate scab control in mature orchards of tall trees is perceived to be difficult, but no data exists to demonstrate the degree of disease control achieved at different heights in the canopy when using conventional, ground-based air-blast sprayers. Experiments were undertaken on 16 m trees of cultivars Desirable (2010, 2011) and Wichita (2011) to compare the severity of scab at different heights on trees receiving fungicide using an air-blast sprayer and non-sprayed trees. There was a consistent difference between treated and non-treated trees in scab severity on fruit to a height of 10 m in early-August, which was slightly less consistent by early-October, six weeks after the last fungicide spray was applied. Spray coverage was studied using water-sensitive cards placed in the canopy of mature pecan trees. Spray coverage was generally uniform up to 10 m, but significantly less surface area was covered by spray at heights >10 m. Thus, in trees receiving fungicide from an air-blast sprayer disease is reduced low in the canopy, but there is little effect in the upper canopy, and the distribution of spray reflects the height to which the severity of pecan scab was reduced. Trees in pecan orchards are often >10-15 m tall, so to achieve adequate scab control additional aerial application of fungicide may be required.

Technical Abstract: Pecan scab (Fusicladium effusum [G. Winter]) is the major disease of pecan in the southeastern U.S., and severe epidemics can result in complete yield loss in susceptible cultivars. Achieving adequate scab control in mature orchards of tall trees is perceived to be difficult, but no data exists to demonstrate the degree of disease control achieved at different heights in the canopy when using conventional, ground-based air-blast sprayers. Experiments on 16 m trees of cultivars Desirable (2010, 2011) and Wichita (2011) compared the severity of scab at 5 different heights (<5.0, 5.0-7.5, 7.5-10.0, 10.0-12.5 and >12.5 m above ground) on trees receiving fungicide using an air-blast sprayer and non-sprayed trees. There was a consistent significant (P<0.0001) difference between treated and non-treated trees in scab severity on fruit to a height of 10 m in early-August, which was slightly less consistent by early-October, six weeks after the last fungicide spray was applied. In additional experiments, spray coverage was studied using water-sensitive cards placed in the canopy of mature pecan trees at 0.0, 5.0, 7.5, 10.0, 12.0 and 15.0 m. Spray coverage was measured as the percent card area covered, and the number of droplets per card using digital image analysis. The spray coverage was generally uniform up to 10 m, but significantly less (P<0.0001) surface area was covered by spray at heights >10 m. Thus, in trees receiving fungicide from an air-blast sprayer disease is reduced low in the canopy, but there is little effect in the upper canopy, and the distribution of spray reflected the height to which the severity of pecan scab was reduced. Trees in pecan orchards are often >10-15 m and older trees can reach >30 m in some orchards, so to achieve adequate scab control additional aerial application of fungicide may be required.