Factors Affecting Reductions in Photosynthesis Caused by Applying Horticultural Oil to Grapevine Leaves

Abstract

Phytotoxicity of horticultural oil, applied shortly before antifungal sulfur, was evaluated for 23 grape cultivars. Oil application significantly reduced accumulation of soluble solids in berries of 9 of 23 cultivars, but there was no relationship with visible foliar injury. Treatment of leaves of Vitis labrusca `Catawba' with 1.5% JMS Stylet-Oil reduced leaf net photosynthesis (Pn) by 50% to 60% and of Vitis vinifera `Chardonnay' by 20% to 30% 1 day after application. Pn was reduced only when the lower (abaxial) leaf surface was treated; treatment of only the adaxial leaf surface had little effect. The Pn depression in `Catawba' persisted 3 to 4 weeks, whereas reductions in `Chardonnay' persisted less than 2 weeks. The Pn-depressing effect of oil was not significantly ameliorated by real or simulated rainfall, and washing the lower leaf surfaces with water and detergent also had only limited effect. There was no significant difference in Pn depression from oil applications made in the middle of the day (stomata open) compared to application in the evening (stomata closed), or from oil applied at higher versus lower application pressure. The greater sensitivity of `Catawba' than `Chardonnay' to Pn depression by oil may be related to the amount of oil retained by the leaves; the pubescent lower leaf surfaces of `Catawba' retained more than twice as much spray emulsion as did the more glabrous leaves of `Chardonnay'. Visible injury was mild in both cultivars, with small water-soaked lesions developing more commonly on `Chardonnay' than on `Catawba' leaves. Spray oil retention data for additional cultivars suggested that differences in retention can explain a portion of the differences in horticultural oil phytotoxicity.