Effects of fungicides and surfactants on the zoospores of Olpidium brassicae

SUMMARY Lettuce big‐vein disease is transmitted from diseased to healthy plants by zoospores of the lettuce root‐infecting fungus Olpidium brassicae. A laboratory technique based on microscopical examination of Olpidium Zoospores is described for assaying the toxicity of chemicals to zoospores. Chemcials found to kill zoospores in <1 h included copper (4 μ/ml), zinc (10μ/ml), diluted preparations of carbendazim (methyl‐2‐yl‐benzimidazole carbamate) as Bavistinand a formulation of Bavisitin containing no carbendazim. Bavistn controlled the disease when introduced at a concentration of 0.6 g/litre into a lettuce crop grown in a re‐circulated film of nutrient. Various surfactants inlcuding Agral, Cetrimide, Deciquam, Ethylan CPX, Hyamine 1622, Manoxol/OT and sodium lauryl sulphate were toxic to zoospores at concentrations of 1–10 μ/ml.