Control of lettuce big‐vein disease by soil sterilisation

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Abstract
SUMMARY: Lettuce plants grown in the glasshouse in big‐vein‐contaminated soil previously treated with methyl bromide at 125–2000 kg/ha (x 0.125– 2.0 the dose used in the field) did not develop big‐vein symptoms, although at dosages up to 500 kg/ha their roots were infected by Olpidium brassicae(Wor.) Dang., the vector of the disease. Soil treatment with dazomet, chloropicrin, 1, 3‐dichloropropene or metham sodium gave symptom‐free plants at doses corresponding to those normally applied in the field. When applied to contaminated field plots, methyl bromide at 750 or 1000 kg/ha controlled the disease, but 10 per cent of plants showed big‐vein symptoms in plots treated at 500 kg/ha. All methyl bromide treatments significantly increased lettuce head weight compared with the untreated controls, and resulted in bromide residues in the plants of 2300–5300 ng/g of dry tissue compared with 30 μg/g in plants from untreated plots. Plants were harvested 12 weeks after completion of methyl bromide application, by which time there was evidence of leaching of bromide from the upper 10 cm of soil.