The effect of sample height on spray coverage in mature pecan trees

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

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/7/2015
Publication Date: 2/1/2015
Citation: Bock, C.H., Hotchkiss, M.W., Cottrell, T.E., Wood, B.W. 2015. The effect of sample height on spray coverage in mature pecan trees. Plant Disease. 99(7):916-925. https://doi.org/10.1094/PDIS-11-14-1154-RE.
DOI: https://doi.org/10.1094/PDIS-11-14-1154-RE

Interpretive Summary: Pecan scab is the most damaging disease of pecan in the southeastern US. Large air-blast sprayers for orchards are used to apply fungicide to control the disease, but little information exists on the spray coverage achieved. In a series of experiments spray coverage at different heights up to 15 m was measured in pecan trees and showed a significantly greater percentage of card area covered at the lowest sample height when compared with the highest sample height. In general, there was little significant difference in spray card coverage up to at least 10 m, but at 12.5 and 15 m, there was significantly less spray coverage compared with the coverage at 5 m. When spray cards were positioned at different heights without possible interference from pecan limbs and foliage, similar effects of sample height on spray coverage were noted. Wind speed measurements showed that air movement declined rapidly with distance from the sprayer fan. At distances >12 m, wind speed was approaching ambient air movement of about 1-3 m sec-1. Characterizing and understanding pesticide spray coverage in pecan will allow us to discern limits imposed by existing technology, and provide the basis for improving spray application methods (and/or tree management) for more efficacious disease control.

Technical Abstract: Pecan scab (caused by Fusicladium effusum) is the most damaging disease of pecan in the southeastern US. Large air-blast sprayers for orchards are used to apply fungicide to control the disease, but little quantitative information exists on the spray coverage achieved in the canopy of these trees. In a series of experiments using water sensitive spray cards to record spray coverage (% area) at different heights up to 15 m, and locations in the canopy of pecan trees showed a significantly greater percentage of card area covered at the lowest sample height when compared with the highest sample height. At the lowest height (5 m) spray coverage on individual cards ranged from 4.7 to 73.5%, and at the highest sample height (15 m), spray coverage ranged from 0.02 to 9.5%. In general, there was little significant difference in spray card coverage up to at least 10 m, but at 12.5 and 15 m, there was significantly less spray coverage compared with the coverage at 5 m. Regression analysis indicated a consistent linear relationship between sample height in the tree and the percentage area covered. When spray cards were positioned at different heights without possible interference from pecan limbs and foliage, similar effects of sample height on spray coverage were noted. Wind speed measurements showed that air movement declined rapidly with distance from the sprayer fan. Whereas at 2 m from the fan, wind speeds were approximately 26 m sec-1 by 10 m they had declined to 2 to 4 m sec-1. At distances >12 m, wind speed was approaching ambient air movement of about 1-3 m sec-1. Although aerial application resulted in numerically greater spray coverage at sample heights >10 m, it was not significant even though a weak linear relationship (R2 = 0.21-0.25) suggested an effect of height. Characterizing and understanding pesticide spray coverage in pecan will allow us to discern limits imposed by existing technology, and provide the basis for improving spray application methods (and/or tree management) for more efficacious disease control.