Effect of tractor speed and spray application volume on spray coverage at different heights in the canopy of tall pecan trees

Authors: Bock, Clive; Hotchkiss, Michael - Mike

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2020
Publication Date: 9/1/2021
Citation: Bock, C.H., Hotchkiss, M.W. 2021. Effect of tractor speed and spray application volume on spray coverage at different heights in the canopy of tall pecan trees. Plant Disease. 105:2509-2520. https://doi.org/10.1094/PDIS-11-20-2420-RE.
DOI: https://doi.org/10.1094/PDIS-11-20-2420-RE

Interpretive Summary: Scab is the most important disease of pecan in the southeastern U.S.A. The yield losses in susceptible cultivars, combined with costs of control amount to tens of millions of dollars annually. Fungicide coverage from sprayers declines with height in the canopy. How application volume (L/ha) and speed (km/h) affect spray coverage at different heights is unknown. Coverage was quantified at heights of 5.0, 7.5, 10.0, 12.5 and 15.0 m in pecan canopies. An orchard air-blast sprayer was operated at 2.4 and 3.2 kph applying 468, 935 or 1871 L/ha. Speeds tested did not affect spray coverage consistently. However, greater volumes resulted in significantly greater spray coverage, but most of that increase was at heights =12.5 m. Although there were significant differences among volumes applied at 12.5 m, they were numerically small. The study demonstrates that higher volumes result in more coverage, but the effect declines rapidly with height. If disease control achieved with 470 L/ha is no different (or is more efficacious) compared to >470 L/ha and is the same at higher speeds (3.2 kph), savings may be possible in terms of operating time and equipment costs.

Technical Abstract: Scab (caused by Venturia effusa) is the most important disease of pecan in the southeastern U.S.A. The yield losses in susceptible cultivars, combined with costs of control amount to tens of millions of dollars annually. It is known that fungicide coverage from air-blast sprayers declines with height in the canopy, and conversely, disease severity increases. But how application volume (L/ha) and speed (km/h) affect spray coverage at different heights is unknown. Coverage was quantified using kromekote cards and Vision Pink™ dye at heights of 5.0, 7.5, 10.0, 12.5 and 15.0 m in pecan canopies. An orchard air-blast sprayer was operated at 2.4 and 3.2 kph applying 468, 935 or 1871 L/ha. Nozzles were selected to provide proportionally similar volumes to the upper and lower canopy positions at set speeds. Speeds tested did not affect spray coverage consistently. However, greater volumes resulted in significantly greater spray coverage, but most of that increase was at heights =12.5 m. Although there were significant differences among volumes applied at 12.5 m, they were numerically small. Card orientation had a profound effect on spray coverage at heights =12.5 m, with most spray being detected on the cards facing horizontally downwards, and least on those facing vertically backwards. The study demonstrates that higher volumes result in more coverage, but the effect declines rapidly with height. If disease control achieved with 470 L/ha is no different (or is more efficacious) compared to >470 L/ha and is the same at higher speeds (3.2 kph), savings may be possible in terms of operating time and equipment costs.