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Tomato blight, a fungal infection called Phytophthora infestans, spreads by wind and water-splash. It also attacks potatoes, and is triggered by warm, wet conditions, making outdoor tomatoes more susceptible than those in a greenhouse. The crop is quickly ruined, and even if it's immediately picked you can't stop the tomatoes rotting.
Solution
Organic
Remove blight-infected plants as soon as they are identified, to prevent spread to other plants.
Chemical
Carry out repeated, preventative spraying the moment the first batch of tomatoes starts to set. Use fungicide containing mancozeb or copper. They don't provide 100 per cent protection, but an attack will be slowed down and you should get ripe tomatoes.
