Eradication of apple powdery mildew (Podosphaera leucotricha) with dormant season sprays of surface‐active agents

Abstract

SUMMARYselection of surfactants (5%, w/v, a.i.) representative of the three main ionic types eradicated Podosphaera leucotricha from buds on dormant apple trees, cvs Cox's Orange Pippin, Jonathan and Michelin.The pathogen was apparently killed without undue damage to buds, although a phytotoxic response in the form of staining of vascular tissue and some crop reductions were observed. Treatments applied with hand lances were equally effective at reducing infections of vegetative buds on terminals, on non‐flowering spurs and flower buds. Indication of a positive correlation between the eradication of mildew from Cox's Orange Pippin trees and an increase in the incidence of apple canker (Nectria galligena) was found at Long Ashton, but this effect was negligible at sites in the West Midlands region.Efficacy of active formulations was reduced by rain soon after application or by a reduction in the concentration of applied material. Applications with a mist‐blower were effective on the cv. Cox's Orange Pippin, but less so on the cv. Jonathan, while on both cultivars efficacy of a spray containing 5 % a.i. decreased in the order 2240:1120:560 1/ha.Anti‐mildew activity was not associated with any particular chemical or ionic type of surfactant, and there was an indication that eradicant activity of alkyl phenol ethoxylates was related to their HLB value.The ecological and economic implications of the use of surfactants as fungicides are discussed.