Effects of adjuvants on distribution and rainfastness of captan sprays on apple leaf scars to control European canker

Abstract

Autumn leaf scars are an important pathogen infection site for European canker on apples. Trials were undertaken to evaluate the effects of adjuvants on spray coverage of new leaf scars, and determine if adjuvants could (1) impart any significant rainfastness to protectant captan sprays and (2) provide any redistribution of captan during a rain event post-leaf drop. Retention on fresh leaf scars was increased with the use of organosilicone-latex sticker adjuvants only. Rain had little effect on spray residues but these adjuvants could improve rainfastness of captan due to their effects on spray retention. Retention on stem wood and around leaf nodes was increased by up to seven-fold with the use of superspreader-type adjuvants, due to redistribution of spray runoff from leaves. High initial deposits resulted in post-rain residues on leaf scars three-fold higher than a captan spray alone. Redistribution of captan onto exposed leaf scars in rain appears likely with these adjuvants.