This fungus disease is notorious because it caused the Potato Famine that killed one and a half million Irish people and causing another million to emigrate. Though best known as a potato disease, it also affects tomatoes, peppers and eggplant. It is called Late Blight because it prefers warmer weather than Early Blight and usually occurs later in the year (it doesn’t usually bother early crops). The spores are most often carried by soil splashed on to leaves by rain or overhead irrigation. They can also be carried on the wind and in the right conditions they can travel long distances rapidly (as happened during the Potato Famine).
This fungus prefers high humidity, wet weather and mild temperatures (50 to 80 degrees F). It first manifests itself as gray-brown necrotic patches on the margins of lower leaves, but these quickly enlarge and kill the whole leaves (sometimes overnight). Brown sunken patches appear on the tubers and may spread into the flesh causing it to rot (or provide entry for other rot causing organisms). The fruit may also rot.
In cool wet weather you should watch for signs of infection and remove any affected plants immediately, but it’s an indication that the plants aren’t happy with the growing conditions. In Western Washington whole beds of Tomatoes and Potatoes died almost overnight. The only thing you can do in these circumstances is dig the tubers 2 weeks after the tops die down and use them. This disease affects yield, but doesn’t affect storability (don’t replant these of course). If growing conditions are always favorable to this disease then its best to use resistant varieties. The following varieties are considered blight resistant, though strains of the disease vary in their virulence and even resistant varieties may not be immune.
Resistant potato varieties include Defender, Cara, Sante, Cosmos, Romano and Jacqueline Lee.
Resistant tomato varieties include: Stupice, Legend, Juliet and Matt's Wild Cherry. Newer varieties specifically bred for resistance include Ferline’ F1 hybrid and 'Fantasio’ F1 hybrid.
To minimize the effects of this disease, make sure the plants have good drainage and air circulation (staking and pruning can help with tomatoes). It is important that leaves don’t stay wet for long periods, so ideally you should use drip irrigation. If you must use sprinklers, water in the morning or early evening, so the leaves can dry out before nightfall. The spores only overwinter in living plants, so remove any volunteers and solanum weeds. Spores will also overwinter in infected seed potatoes, so you should only plant certified disease-free seed potatoes.
Image: Florida Division of Plant Industry Archive, Florida Department of Agriculture and Consumer Services, Bugwood.org