THE REPRESSION AND STIMULATION OF GROWTH OF ERYSIPHE SP ON RHODODENDRON BY FUNGICIDAL COMPOUNDS

Erysiphe sp. is a causal agent of powdery mildew on Rhododendron. A no vel in vivo method permitting the screening of fungicides on woody pla nts is described. Eight fungicides were evaluated for activity against Erysiphe sp. using Rhododendron ponticum microplantlets grown in vitr o. Pathogen development changed with both the type of fungicidal compo und and the concentrations applied. The most active materials were fen propidin and penconazole, which showed high activity at the lowest con centrations. Six of the compounds performed more effectively than a mi xture of bupirimate + triforine (Nimrod T), the standard recommendatio n for control of this pathogen on Rhododendron. All fungicides affecte d the sporulation of Erysiphe sp., with propiconazole, pyrazophos and triadimenol causing a significant increase in sporulation at the lowes t concentrations. At higher concentrations, sporulation was significan tly reduced by all treatments. No phytotoxic effects were detected wit h any fungicide at any concentration. The growth of plantlets in most treatments showed no significant difference from the untreated control s. The results of the study are discussed in relation to strategies fo r control and the epidemiology of Erysiphe sp. infecting Rhododendron.