Advice from Thompson & Morgan

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.
News article

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.

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.

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