Advice

RHS Help & Advice

Tomato blight

Hosts

Tomatoes, potatoes and relatives, especially Solanum species.

Symptoms

Brown, rapidly spreading lesions on leaves and stems. Firm brown spreading patches of rot on the fruit. Infected tissues die rapidly.

Biology

Caused by the fungus Phytophthora infestans. This produces sporangia on infected tissues which are dispersed by wind and rain splash. Warm, wet conditions are needed for their production and also for infection to occur. The sporangia release infective zoospores which penetrate into the host tissues.

Resting spores produced in dead plant tissue can overwinter in the soil to initiate infections the following season. However, the risk from this source of the disease is small compared with infection arising from air-borne sporangia. Infected potato tubers left in the ground from the previous year, or dumped at the edge of vegetable plots, are important infection sources for the following year’s crops. Potatoes are usually infected before tomatoes.

Control

Tomatoes grown under glass are not always infected. Plants likely to be attacked - especially outdoor tomatoes - require protective sprays of mancozeb (Dithane) or copper (Murphy Traditional Copper or Vitax Bordeaux Mixture). Apply these before the symptoms are seen as a protection against attack. They will not totally prevent infection, but often slow it sufficiently to save the crop.

The fungus can be seed borne, so do not save seed from infected fruit. Destroy infected plants - do not compost them. There is very little resistance in most cultivars, but 'Ferline', ‘Legend’ and ‘Fantasio’ are believed to be resistant.

More information on potato blight

< Back to advice archive