Evaluation of adjuvants for enhanced management of Podosphaera xanthii using potassium silicate and biocontrol agents

Abstract

Three adjuvants [polyether-polymethylsiloxane-copolymer (Break-Thru®), alkoxylated fatty alkylamine polymer/ethoxylated sorbitane ester (Partner 650®) and polyoxyethylene 20 sorbitan monooleate (Tween-80®)] were compared for their ability to improve the efficacy of potassium silicate and biocontrol agents (BCAs) against powdery mildew of zucchini caused by Podosphaera xanthii (Castagne) U. Braun & Shishkoff. The most effective adjuvant was investigated for its direct impact on the pathogen and deposition of BCAs using electron microscopy and its compatibility with the BCAs and zucchini plant was evaluated to establish an integrated management of powdery mildew. Furthermore, effects of Si spray in accumulation of Si and K in the leaves were measured. Break-Thru® and Partner 650® improved the efficacy of silicate by 18–35%. The best adjuvant, Break-Thru®, caused a collapse and degeneration of the conidia and hyphae of the pathogen, restricted growth and spread of the powdery mildew colonies by binding its propagative structures, and improved deposition of BCAs on the leaf surface and the fungus. Break-Thru® was compatible with the plant and the BCAs at concentrations <0.4 ml l−1, and can be used with selected BCAs and SiO2 for an integrated management of powdery mildew. SiO2 applied as a foliar spray increased the level of Si in zucchini leaves, which may contribute to systemic resistance of the plant against the disease.