Microscopic analysis reveals potential mode of action of foliar-applied potassium silicate against powdery mildew development

Abstract

Silicon (Si) efficiently controls Podosphaera xanthii in melon by affecting epidemic components related to the infection and colonization processes. Root amendments of Si prime melon defences, but its mode of action when sprayed on the leaves is unknown. The effects of potassium silicate (PS) on the development of the pathogen were observed by scanning electron microscopy. Both forms of PS application reduced conidial germination and delayed colony development. At 120 h after inoculation (hai), differentiation of the conidiophores was inhibited only by the root treatment and at 144 hai visual differences between PS treatments accentuated. Comparison of foliar PS with compounds of contrasting pH and ionic strength (KOH, KCl and PEG) showed that only PS and PEG inhibited conidial germination ruling out these variables as inhibitory factors. Alteration of the leaf surface tension was also excluded since it was affected by all compounds. The results indicate that foliar-applied PS affects P. xanthii conidial germination of and colony development by modifying the osmotic potential of the leaf surface.