Tomatoes are related to the potato, both being members of the solanaceae group of plants, and as such, are vulnerable to similar pests and diseases.
Blight is a serious disease for outdoor tomatoes, but is not as common on those grown under cover as the cover can deny access to the air-borne spores.
Potatoes are usually infected before tomatoes and are often the source of the spores.
Symptoms and Causes:
Tomato Blight - Phytophthora infestans is the main fungal disease of tomatoes and is caused by spores that are dispersed by the wind.
The infection is seen as brown, rapidly spreading lesions on leaves and stems, and patches of rot on the fruit.
The disease is more prevalent during wet and or warm humid weather.
Maintaining good air circulation in the greenhouse will help to keep humidity under control, and / or using an electric fan to create air movement will also help.
This website can be used to track periods when blight is most likely to strike.
As mentioned above, crops grown under cover are less prone to the disease but as a preventative measure they should be sprayed at the same time as 'outdoor' cultivars are being sprayed.
Spray with a suitable copper based fungicide before symptoms occur and every ten days thereafter.
Spraying will not prevent infection, but may slow it down and save the crop.
The fungus can be seed borne, so do not save seed from infected fruit and destroy all infected plants and do not compost.
Purchasing blight resistant varieties can also help.