Late blight is a serious and widespread disease of the Solanaceae family. It is often called potato blight or tomato blight as it particularly affects these crops, and can destroy a tomato or potato crop in as little as 10 days. However the causal pathogen is the same. This destructive fungal disease is caused by spores of Phytophthora infestans which are spread on the wind and may also contaminate potato tubers in the soil. The severity of this disease was seen in the 1840s when a succession of potato blight epidemics led to the Irish Potato Famine.
Late blight is particularly prevalent during warm humid weather and can be especially problematic in late summer during wet weather. In the UK, tomato and potato blight may occur as early as June in the south. This disease is most damaging to outdoor crops, but can also affect greenhouse crops of tomatoes if conditions are humid.
Dark brown blotches appear on the leaves, particularly towards the leaf tips and edges. White fungal spores develop around these lesions on the undersides of the leaves, and further lesions develop on the stems. Leaves and stems rapidly blacken and rot causing plant collapse.
The spores are released on the wind and quickly spread to infect neighbouring plants. Spores may also be washed down into the soil where they can infect potato tubers causing a red-brown rot directly beneath the skin which slowly spreads towards the centre of the tuber. In tomatoes, similar symptoms of blight occur to the foliage and damage to the fruits is identifiable as they turn brown, and then shrivel and decay.
The fungal spores of late blight overwinter in old potato tubers that remain in the ground or on the compost heap. These tubers may grow the following year to produce infected shoots. Fresh fungal spores of blight are then released on the wind to infect new crops. Spores may also be washed into the ground by heavy rainfall to infect tubers growing there.
Prevention of late blight is important as there is little that can be done to save an infected crop, so it is well worth taking these precautions: