Sprayed tomato plant with contact fungicide 'Dithane' (80% w/w mancozeb) to protect against tomato blight (Phytophthora infestans). Still a little early - blight has always attacked my tomotoes in August. As outlined last year (see 'Tomato Blight Update', 26 August), it requires '. . . consecutive 24 hour periods, each with a minimum temperature of 10 deg C (50 deg F) and each with a minimum of at least 11 hours where the relative humidity is 89 per cent or more'. i.e. hot muggy weather.
Contact fungicides are effective over a much wider range of diseases than systemic ones, but must be applied BEFORE disease spores come in contact with plant. They work by preventing spore germination on plant surface and need applying every two or three weeks. Systemics prevent spore germination but also block active metablolism inside plant - so should also be effective AFTER infection has occurred. See also: 'How Fungicides Work'.
Not sure if 'Systhane' (myclobutanil) can be used on tomatoes. It can be a bit fierce. Tested on a single leaf, which seemed to change colour, though not sure if this was due to chemical or to other causes. Bordeaux mixture (copper sulphate) is also effective on potatoes, though again not sure if it can be used on tomatoes. Does anyone know?
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