A comparison of ground-based air-blast sprayer and aircraft application of fungicides to manage scab in tall pecan trees

Authors: Bock, Clive; Hotchkiss, Michael - Mike

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/23/2020
Publication Date: 4/22/2020
Citation: Bock, C.H., Hotchkiss, M.W. 2020. A comparison of ground-based air-blast sprayer and aircraft application of fungicides to manage scab in tall pecan trees. Plant Disease. 104:1675-1684. https://doi.org/10.1094/PDIS-11-19-2345-RE.
DOI: https://doi.org/10.1094/PDIS-11-19-2345-RE

Interpretive Summary: Pecan scab is caused by the plant pathogenic fungus, Venturia effusa. It is the most destructive disease of pecan in the southeastern USA, and elsewhere. In scab-prone areas, susceptible cultivars must be sprayed with fungicide every 2 to 3 weeks to ensure yield and kernel quality. The most widely used method to apply fungicide is large orchard air-blast sprayers. Pecan trees grow tall, and air-blast applied sprays result in a gradient in spray coverage and thus of scab, which increases in severity as coverage declines. Aerially application of sprays is also practiced, but the disease distribution and spray coverage of these two methods has not been compared or contrasted which will provide valuable information to growers wishing to make decisions regarding preferred spray application methods. We compared air-blast, aerial, and air-blast+aerial applications for efficacy controlling scab at 5 heights in the canopy of 25 m ‘Schley’ pecan trees. Results showed that in non-treated control trees there was decline in severity with height. Trees receiving air-blast had less disease than control trees at heights <12.5 m. Scab severity in trees receiving the air-blast treatment generally had a positive linear relationship with tree height. Aerial application frequently resulted in lower severity at all sample heights compared to the non-treated control, even at heights of 5.0 to 7.5 m, and the difference compared to the air-blast treatment was not always significant at these heights. There was a negative linear relationship between scab severity and sample height on aerially treated trees. The combination of air-blast+aerial treatments resulted in uniformly low severity of scab at all heights. Spray coverage measured on water sensitive cards indicated a declining gradient with height when applied using the air-blast sprayer, but aerial applications resulted in uniformly low spray coverage at all sample heights. Both air-blast and aerial sprays resulted in control of scab at most heights, although air-blast sprays tended to fail at heights >12.5 m, and aerially-treated trees tended to show a slight increase in disease severity in the lower canopy. These results provide a basis for understanding the relative efficacy and advantages of different methods for fungicide spray application in tall pecan trees and pave the way for further research to better understand the impact of frequency and timing of these two methods.

Technical Abstract: Pecan scab (caused by Venturia effusa) is the most destructive disease of pecan in the southeastern USA, and elsewhere. In scab-prone areas, susceptible cultivars must be sprayed with fungicide every 2 to 3 weeks to ensure yield and kernel quality. The most widely used method to apply fungicide is large orchard air-blast sprayers. Pecan trees grow tall, and air-blast applied sprays result in a gradient in spray coverage and thus of scab, which increases in severity as coverage declines. Aerially application of sprays is also practiced, but the disease distribution and spray coverage of these two methods has not been compared or contrasted which will provide valuable information to growers wishing to make decisions regarding preferred spray application methods. We compared air-blast, aerial, and air-blast+aerial applications for efficacy controlling scab at 5 heights in the canopy of 25 m ‘Schley’ pecan in 2013 and 2014. Results showed that in non-treated control trees there was a negative linear relationship between scab severity and sample height, with scab on fruit being most severe at most heights compared to other treatments, except in the upper canopy >12.5 m, where severity in trees receiving the air-blast spray was most often similar. However, severity at <12.5 m was often lower in the trees receiving the air-blast spray compared to the non-treated control. Scab severity in trees receiving the air-blast treatment generally had a positive linear relationship with tree height. Aerial application frequently resulted in lower severity at all sample heights compared to the non-treated control, even at heights of 5.0 to 7.5 m, and the difference compared to the air-blast treatment was not always significant at these heights. There was a negative linear relationship between scab severity and sample height on aerially treated trees. The combination of air-blast+aerial treatments resulted in uniformly low severity of scab at all heights, which although often numerically lower, was rarely significantly different to the single treatments. There was little or no relationship between scab severity and sample height. Spray coverage measured on water sensitive cards indicated a declining gradient with height when applied using the air-blast sprayer, but aerial applications resulted in uniformly low spray coverage at all sample heights. In conclusion both air-blast and aerial sprays resulted in control of scab at most heights, although air-blast sprays tended to fail at heights >12.5 m, and aerially-treated trees tended to show a slight increase in disease severity in the lower canopy. These results provide a basis for understanding the relative efficacy and advantages of different methods for fungicide spray application in tall pecan trees and pave the way for further research to better understand the impact of frequency and timing of these two methods.