Scab (caused by Venturia effusa) is the major yield limiting factor of pecan in the southeastern U.S.A. Control is typically achieved using conventional fungicides applied using an air-blast sprayer. Generally, sprays are applied to both row sides simultaneously by travelling up and down the center line of consecutive rows (every row middle, ERM). Early in the season some growers have used alternate row middle (ARM) spray applications. However, the impact of early season ARM on scab control and fruit weight has not been explored. In this study ARM every 7 and 14 days, and ERM every 14 days for the first six weeks (and ERM in all treatments thereafter) were compared to non-treated controls in the 2018 and 2019 seasons. There were some effects of spray date and tree side on spray coverage that were likely due to canopy development after bud break, and the ERM application had at least twice the spray coverage in both seasons compared to the ARM applications. In 2018 scab severity on young leaflets was least on the ARM14 treatment compared to the other treatments, but in 2019 all three treatments had less severe scab when compared to the control (but were not different to each other). For the older leaflet assessment in both seasons there was more severe scab on leaflets of the control compared to the ARM7 treatment, and in 2019 compared to the ERM treatment as well. Other treatments were not different to the control. In both years the severity of scab on fruit on the control was significantly greater than on fruit on any of the treated trees (but differences among treatments were small and inconsistent between seasons). In both seasons there were effects of tree height on scab severity, and harvested fruit were consistently heavier on trees receiving the ERM sprays throughout the season. In this study ARM sprays early in the season did not have large or lasting effects on scab control on leaflets or fruit, but the season long ERM treatment appeared to confer a slight benefit to fruit weight.
Bock et al. (Sun,) studied this question.