Downy mildew disease of the cultivated lettuce

Abstract

1. Lettuce seedlings, grown from both ordinary commercial seed and seed from mother plants known to be infected with mildew, almost invariably failed to show symptoms of the disease under conditions favourable to mildew development. There was thus negligible evidence of the seed-borne transmission of mildew. 2. Oospores were not found by the microscopical examination of debris from infected plants, and seedlings grown in soil containing a high proportion of such debris for a period considered to be adequate, under favourable conditions for the development of mildew, did not become infected. The transmission of the disease by contaminated soil is therefore unlikely. 3. Attempts to transmit mildew from composite weeds to lettuce were successful only with some wild species of Lactuca . These species are probably too rare to be important in the spread of the disease. 4. The survival of mildewed lettuce was lower, and their date of maturity later, than that of healthy plants. This, together with the disfigurement caused by the disease, gives it its commercial importance. 5. A number of commercial lettuce varieties commonly grown in England showed varietal differences in their susceptibility to the pathogen but these differences were not sufficiently marked to make the exclusive cultivation of the more resistant varieties worth while as a control measure. 6. 2:2:40 Bordeaux mixture, Excess-Lime Bordeaux (2:10:40), an orthophosphate copper spray, and Cuprocide 54 (1 lb. to 50 gal.) all gave adequate protection from mildew disease but sprays containing no copper such as lime sulphur and pentachloronitrobenzene did not. All the copper- containing sprays are liable to produce phytocidal damage, particularly in cold weather. 7. Fungicidal vapours such as benzol and paradichlorbenzene were troublesome to use and gave a poor control of lettuce mildew. 8. Bremia conidia under favourable temperature conditions germinated readily in saturated lime solution and were also found to germinate more readily in slightly alkaline than in slightly acid solution.