AbstractThe vapour phase and protectant activities of members of seven homologous series of alkyldinitrophenols against cucumber powdery mildew (Oidiumsp.) are discussed. No correlation existed between vapour activitiesin vitroorin vivoand vapour pressure, but a positive correlation between Vapour' protectionin vivoand conventional protectant activity was evident. Although vapour activity occurs with some homologues, results obtained for zone assaysin vivoare probably better explained in terms of easy movement of the compound in the leaf surface than in terms of vapour transmission. Alkyldinitrophenyl‐crotonates showed no zone activityin vivobut often good protectant activity. The possible existence of two optimum π‐values for protectant activity is suggested. No appreciable systemic activity was found with these compo