On Crops: Tomatoes, potatoes
Worldwide, especially in cool temperate climates
Following any extended period of cool rain, or in very moist greenhouse conditions, tomato leaves suddenly develop pale wilted patches that are damp rather than dry. The disease spreads very quickly and can defoliate previously healthy plants in about a week. Distinctive dark brown patches often appear on stems and may spread to tomato fruits or potato tubers. The most favourable temperature range for late blight development is 60-75F (15-24C). Leaf symptoms on potato are similar to those on tomato, but the disease is slightly slower to spread to new plants.
Tomato or potato plants that have lost their leaves to late blight are poor producers. When tomato plants survive and go on to produce fruits, the fruits often have leathery patches that do not ripen properly. If not dug up early, blight spreads to potato tubers causing red-brown decay which will eventually rot and prevent them storing well.
Try resistant tomato or potato varieties, which are new but very worthwhile. Rotate your tomatoes and potatoes to fresh sites every three years, and use mulches to keep rain from splashing onto foliage. In some areas the risk of late blight is so high that tomatoes must be grown in greenhouses. When used preventatively, organic fungicides including copper and strains of Bacillus bacteria can be helpful managing this disease. In areas where late blight is common, carefully inspect volunteer tomato and potato plants for signs of disease before allowing them to grow in your garden.
Most of the time it is best to pull up plants and compost them. However, varieties differ in their susceptibility, and some will outgrow the disease and produce a small crop. Potatoes from blighted plants can be eaten, but should not be stored.