Late Blight Prevention

Symptoms

• Check tomato plants for brown, oily-looking, irregularly shaped lesions, often surrounded by yellowing tissue and white fungal growth, on leaves and stems. The fruit may have hard or rotting patches.

  On potato leaves and stems, look for dark lesions with a gray-green halo and whitish fuzz. On the tuber you may see brown or purple lesions on the skin, with granular, red-brown rot extending half an inch into the flesh.

  Infected plants start to wither and lose leaves within a few days.

How Blight Spreads

• The source of an outbreak is often an infected tuber or tomato plant left to rot outdoors. Spores disperse on wind and rain or during harvesting, handling or storage, and can survive for weeks in soil. Warm, humid weather helps them develop. Winter frost and temperatures over 80 degrees Fahrenheit usually kill the spores, but evolving genotypes appear able to withstand increasingly high temperatures. Late blight damage can let other, bacterial diseases take hold, sometimes masking the symptoms of blight until too late. Garden petunia and wild-growing bittersweet nightshade may also harbor late blight.

Prevention

• Choose resistant varieties of tomato and certified seed potatoes. Seek up-to-date advice on new varieties from a university extension or county diagnostic service.

  Plant tomatoes in full sun, well spaced to allow air to circulate freely. Prune excess foliage and water the base, not the leaves.

  Keep looking out for trouble. Inspect your crops every few days. Check local forecasts that predict environmental conditions favoring blight and use spreadsheets available online to monitor and report occurrences. From June onwards, be ready to spray regularly with preventative fungicides. Seek advice on brands, as resistance soon develops, and follow the manufacturer's recommendations on rates of application. Only copper-based sprays are acceptable in organic food production.

Treatment

• If you find or suspect late blight in garden tomatoes or potatoes, pull out the plants, including roots. Cover them in black plastic sacks or tarpaulin and leave them in the sun for a few days to kill the spores by heat. Burn the plants or put them in the trash, not the compost bin. When you cull surplus or reject potatoes, or pull up self-seeded strays, destroy them before they become a breeding ground for late blight.

  Sanitize gardening tools in a bleach solution, and take care not to transmit spores on your hands or clothing.