Disruption of chlorophyll organization and function in powdery mildew-diseased cucumber leaves and its control by the hyperparasite Ampelomyces quisqualis

The hyperparasite Ampelomyces quisqualis is a potential biological control agent for powdery mildew (PM) disease. Cucumber plants were exposed to PM with and without subsequent A. quisqualis treatment. These plants were examined in comparison to healthy uninfected plants. Disease symptoms, including chlorosis and necrosis, were most prominent on the plants exposed only to PM. Electron micrographs of leaf sections of these diseased plants indicated marked deterioration in the morphological organization of chloroplast membranes. In comparison, chloroplasts of A. quisqualis-treated plants seemed undamaged and like those of uninfected plants. Low-temperature fluorescence emission spectra of diseased leaf tissue showed a dramatic increase in the far-red:red emission ratio in comparison with uninfected and A. quisqualis-treated leaves. Room-temperature fluorescence transients revealed reduced variable fluorescence in the diseased plants. Both sets of fluorescence data indicate a disease-correlated increase in levels of uncoupled chlorophyll. Fluorescence methods, thus, are proposed as ideal means for assessing hyperparasite effectiveness in controlling PM