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The fungus that causes this problem spreads rapidly in the warm, humid conditions of a greenhouse. It usually becomes apparent on the lowest tomato leaves from early or midsummer, which develop yellow blotches on the upper leaf surface. The leaves eventually turn brown and wither. If the problem isn't tackled, tomato yields are lower and the fungus can overwinter in the greenhouse and persist the following year.
Solution
Organic
Good ventilation in the greenhouse helps, with plants well spaced out. Avoid splashing the leaves when watering. Promptly remove and get rid of any infected leaves and destroy all the plants at the end of the season. Disinfect the entire greenhouse and choose resistant varieties such as 'Dombello' and 'Shirley'.
Chemical
Spray infected plants with mancozeb.
