Apple scab management assisted by postharvest tree defoliation with vegetable oil emulsion

Abstract

Vegetable oil emulsion (VOE) was applied to 'Gala' and 'Fuji' apple (Malus xdomestica) trees after harvest to hasten defoliation and reduce apple scab (Venturia inaequalis). Applied at 2%, 4%, or 6%, VOE applied to whole trees in the fall induced leaf drop, with the highest concentration causing the most defoliation. At the same concentration, VOE applied in early, mid, or late October had similar effects on leaf drop. VOE treatment reduced respiration and stimulated ethylene production in shoot tissue within 24 hours of application. None of the treatments affected tree hardiness during the winter, or shoot growth the following spring. Return bloom density was unaffected; however, VOE tended to delay anthesis by 2 to 5 days. Under controlled conditions, 'Gala' and 'Fuji' trees inoculated with scab spores developed 48% and 65% scab, respectively. VOE-induced defoliation reduced scab by 50% to 65%. VOE-induced defoliation plus manual leaf removal from the orchard floor, or VOE-induced defoliation in late fall (15 Oct.-15 Nov.) plus application of 5% lime sulfur in early spring, controlled scab to <5% on both leaves and fruit. Neither lime sulfur nor urea applied in late fall at 2% induced defoliation or controlled scab. VOE at 4% plus 2% lime sulfur and/or 2% urea applied in late fall, however, defoliated 'Gala' trees effectively and controlled scab on fruit to <7% the following spring. In the 'Fuji' planting, the combination of 4% VOE plus 2% lime sulfur and 2% urea reduced scab on fruit from 21% in controls to 0%.