SUMMARYWhen mildew-inoculated cucumber leaves (Sphaerotheca fuligineaonCucumis sativusvar. Ashley) were immersed in water for 1 sec to 20 h at 0 to 80 h after inoculation, and then returned to standard greenhouse conditions, the mildew covered the leaf sooner, sporulated sooner and produced more conidia than control unwetted leaves. If immersion was for 20 to 50 h duration, there was usually no consistent increase or decrease in mildew development. Immersions of 50–70 h were usually inhibitory to the mildew, but not injurious to the host. Immersions of more than 70 h were usually lethal to the mildew, and immersions of more than 100 h were injurious to the leaves. Spraying inoculated leaves with water at low impact pressure also increased mildew, but at high impact pressure it prevented infection or cured infected plants. Powdery mildews of bean, barley, dock and zinnia were similarly tolerant of water, but were not clearly stimulated by water.