Grapevine powdery mildew, caused by the fungus Erysiphe (Uncinula) necator, is a major disease affecting grape yield and quality worldwide. In conventional vineyards, the disease is controlled mainly by regular applications of sulfur and synthetic fungicides and, in organic agriculture, by sulfur and botanical and mineral oils. Research has identified milk and whey as potential replacements for synthetic fungicides and sulfur in the control of powdery mildew. Electron spin resonance and scanning electron microscopy were used to investigate the possible mode or modes of action of milk and whey in the control of powdery mildew. Electron spin resonance experiments showed that various components of milk produced oxygen radicals in natural light, which may have contributed to the reduction of severity of powdery mildew on treated leaves. Milk and whey caused the hyphae of E. necator to collapse and damaged conidia within 24 h of treatment. Hydrogen peroxide, applied as a source of free radicals, also caused collapse of the hyphae of E. necator but did not damage conidia, and appeared to stimulate germination. Lactoferrin (an antimicrobial component of milk) ruptured conidia, but damage to hyphae was not evident until 48 h after treatment. The results support the hypothesis that free radical production and the action of lactoferrin are associated with the control of powdery mildew by milk.