Potato and tomato blight is a disease caused by a fungus-like organism which spreads rapidly in the foliage and tubers or fruit of potatoes and tomatoes in wet weather, causing collapse and decay.
It is a serious disease for potato and outdoor tomatoes, but not as common on tomatoes grown in greenhouses.
Blight is specific to tomatoes and potatoes, and some ornamental relatives of these two crops are also susceptible. Cases have been recorded on some ornamental Solanum species (e.g. S. laciniatum), and also on Petunia.
For full details visit the RHS Potato and Tomato Blight page.