Late blight (Phytophthora infestans) affects both tomatoes and potatoes. The disease survives over winter on volunteer potatoes (plants growing from tubers left in the ground).
Spores can also blow in from nearby infected crops. In warm, wet seasons the disease spreads rapidly – it’s less of a problem in fine weather. Commercial potato growers use ‘Smith periods’ to predict when their crops are at risk (two successive days with temperatures above 10°C and relative humidity above 90% for more than 11 hours each day). You can see when these are forecast here: Blightwatch. The only materials available to gardeners for the control of blight contain copper and are ‘protectants’ which means they have to be applied before the plants are infected, with coverage of all surfaces. Copper is toxic to aquatic organisms and it accumulates in the soil so we don’t use it in our garden.
Did You Know?
British Tomato growers use an army of over 40 million beneficial predatory bugs called Macrolophus in summer to keep pests out of their crops, reducing the need for chemicals.