Fungicide spray coverage from ground-based sprayers in mature pecan trees

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

Submitted to: Phytopathology
Publication Type: Abstract Only
Publication Acceptance Date: 5/13/2013
Publication Date: 6/1/2013
Citation: Bock, C.H., Hotchkiss, M.W., Cottrell, T.E., Wood, B.W. 2013. Fungicide spray coverage from ground-based sprayers in mature pecan trees. Phytopathology. 103:S2.17 http://apsjournals.apsnet.org/loi/phyto.

Interpretive Summary: Air-blast sprayers are widely used to control pecan scab (caused by the fungus Fusicladium effusum) on pecan trees. Good spray coverage is critical to ensure disease control and to minimize risk of fungicide resistance. Spray coverage from an air-blast sprayer, typical of the sprayer used by commercial producers, was measured with water sensitive cards placed at different heights in the canopy and when suspended from helium-filled weather balloons to avoid the effect of foliage obstructing sprays. In trees, spray coverage was generally uniform up to 10 m. On strings, set card orientation (face up or down) affected coverage, but was generally consistent up to 10 m. Less surface area was covered by spray at heights above 10 m. Spray coverage was greater on cards attached to the string compared to the trees, suggesting foliage reduced the effectiveness of spray coverage in the canopy. Thus, spray coverage was reduced with spray height, confirming some previously reported effects of height on spray and disease gradients in pecan. Considering trees in young orchards are often >10-15 m and older trees in mature orchards can be >30 m adequate scab control may require additional aerial application.

Technical Abstract: Air-blast sprayers are widely used to control pecan scab (Fusicladium effusum) on pecan trees. Good spray coverage is critical to ensure disease control and to minimize risk of fungicide resistance. Spray coverage from an air-blast sprayer, typical of the sprayer used by commercial producers, was measured with water sensitive cards placed at different heights in the canopy (0, 5, 7.5, 10, 12.5, 15 m), and when suspended from helium-filled weather balloons (0, 5, 10, 15 and 20 m) to avoid the effect of foliage obstructing sprays. Spray coverage was measured as the percent card area covered, and the number of droplets per card using digital image analysis (Assess V2.0). In trees, spray coverage was generally uniform up to 10 m. On strings, set card orientation (face up or down) affected coverage, but was generally consistent up to 10 m. Less surface area was covered by spray at heights above 10 m. The numbers of droplets could not always be accurately estimated at heights <10 m due to droplet overlap. Spray coverage was greater on cards attached to the string compared to the trees, suggesting foliage reduced the effectiveness of spray coverage in the canopy. Thus, spray coverage was reduced with spray height, confirming some previously reported effects of height on spray and disease gradients in pecan. Trees in young orchards are often >10-15 m and older trees can reach >30 m in mature orchards so adequate scab control may require additional aerial application.