Non-pathogenic Powdery Mildews Induce Resistance in Pea (Pisum sativum) against Erysiphe pisi

Inoculation of pea (Pisum sativum) with pea nonpathogenic powdery mildews (PNPM) (Oidium sp., Phyllactinia corylea and P. delbergiae) induced resistance against a subsequent infection with Erysiphe pisi. Conidial germination, appressorium formation and secondary branch development in E. pisi conidia on pea plants were reduced in the presence of the three PNPM compared with uninoculated host plants. Microscopical observations also revealed that there was no hypersensitive cell death or lignin formation in the pea leaf tissues following treatment with the three PNPM. Reduced disease intensity (DI) of pea powdery mildew was observed up to 12 days after inoculation of the three PNPM. Pre-inoculation with P. corylea led to the highest disease suppression (21.2% DI) compared with non-inoculated control plants (96.5% DI). Highperformance liquid chromatographic (HPLC) analysis of different parts of pea plants with and without PNPM inoculation revealed increased synthesis of total phenolics and also of gallic, tannic and ferulic acids. However, the amount of phenolic compounds decreased in leaves treated singly with E. pisi or in combination with the PNPM.