Effect of Tractor Speed and Spray Application Volume on Spray Coverage at Different Heights in the Canopy of Tall...

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 (liter/ha) and speed (km/h) affect spray coverage at different heights is unknown. Coverage was quantified using Kromekote cards (CTI Paper USA, Sun Prairie, WI) and Vision Pink dye (GarrCo Products, Converse, IN) 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 km/h applying 468, 935, or 1,871 liters/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, differences 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 downward, and least on those facing vertically backward. The study demonstrates that higher volumes result in more coverage, but the effect declines rapidly with height. If disease control achieved with 470 liters/ha is no different (or is more efficacious) compared with >470 liters/ha and is the same at higher speeds (3.2 km/h), savings may be possible in terms of operating time and equipment costs.