Effect of tractor speed and spray application volume on severity of scab and fruit weight at different heights in the canopy of tall pecan trees

Authors: Bock, Clive; WELLS, LENNY - University Of Georgia; Hotchkiss, Michael - Mike

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2021
Publication Date: 12/1/2021
Citation: Bock, C.H., Wells, L., Hotchkiss, M.W. 2021. Effect of Tractor Speed and Spray Application Volume on Severity of Scab at Different Heights in the Canopy of Tall Pecan Trees. Plant Disease. Vol 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 a disease of pecan caused by a fungal pathogen, and is the most important yield-limiting disease of pecan in the southeastern USA. The disease is managed using fungicides, but spray coverage is an issue in tall trees. In four experiments we used an air-blast sprayer to compare scab severity on fruit at 5 to 15.0 m height in trees receiving the same dose of fungicide at 468, 935 and 1871 L/ha at 2.4 and 3.2 km/h (in two additional experiments fungicides were applied at 4.0 km/h @ 470 L/Ha, 4.0 km/h @ 940 L/Ha and 4.0 km/h @ 1120 L/Ha). in cv. Schley in 2015, 2016 and 2017, and on cv. Desirable in 2017, 2019 and 2020. All treatments reduced scab compared to the control. There was no consistent difference among the treatments for severity of scab on foliage, immature fruit, or mature fruit at any height up to 15.0 m (maximum height sampled). Estimated cost and water savings based on faster speed and lower volume were considerable. These preliminary observations indicate in most seasons, low volumes and faster speeds may be similarly efficacious to high volumes and slower speeds for controlling scab in tall pecan trees, and offer greater resource use efficiency.

Technical Abstract: Scab (caused by Venturia effusa) is the most important yield-limiting disease of pecan in the southeastern USA. On susceptible cultivars, the disease is managed using fungicides, but spray coverage is an issue in tall trees. In four experiments we used an air-blast sprayer to compare scab severity on fruit at 5 to 15.0 m height in trees receiving the same dose of fungicide at 468, 935 and 1871 L/ha at 2.4 and 3.2 km/h (in two additional experiments fungicides were applied at 4.0 km/h @ 470 L/Ha, 4.0 km/h @ 940 L/Ha and 4.0 km/h @ 1120 L/Ha). An air-blast sprayer was used for the applications, which included typical recommended active ingredients (a.i.). Nozzles were selected to provide similar proportions of spray to the upper and lower canopy. The treatments (or subsets thereof) were repeated in 2015 to 2017 on cv. Schley, and in 2017, 2019 and 2020 on cv. Desirable. All treatments reduced scab compared to the control. Overall, there was no consistent difference among the treatments for severity of scab on foliage, immature fruit, or mature fruit at any height in the canopy up to 15.0 m (maximum height sampled). Fungicide applied at 2.4 or 3.2 km/h @ 468 L/ha was as effective at reducing disease as were the higher volumes (sometimes more so). The scab epidemic severity affected control efficacy. Estimated cost and water savings based on faster speed and lower volume were considerable. These preliminary observations indicate no single volume or speed was consistently superior to control scab; this suggests in most seasons, low volumes (high concentration of a.i.) may be similarly efficacious as high volumes (low concentration of a.i.) for controlling scab in tall pecan trees, and offer greater resource use efficiency.