Factors in testing fungicides against powdery mildews. II. Tests against oat powdery mildew in the greenhouse

SUMMARY Some factors affecting fungicidal efficiency were studied during greenhouse tests with Erysiphe graminis on oat seedlings. In a comparison of non‐ionic, cationic and anionic surface‐active compounds, for formulating chemicals for testing, 'Ethylan CP' at 0·0025% in demineralized water at 30°C., plus 1–2% of acetone or ethyl alcohol, was found satisfactory and was used throughout later work. Studies of the effect of time of eradicant application on the efficacy of 'dinocap', lauric acid and a captan analogue demonstrated that these chemicals applied 24 hr. after inoculation were significantly less effective than at 48 and 72 hr. Fungitoxicity trials using a homologous series of n‐alkyl fatty acids and related compounds showed that lauric acid applied at 2·5 X 10‐4m as an eradicant 48 hr. after inoculation gave 99% reduction of infection. Of the related compounds, n‐alkylamines with 11–13 carbon atoms equalled the acids in activity, but the amides and fatty alcohols at the same molar concentration were inactive.