Identifying Late Blight on Tomato Plants

Late blight is a fungal disease that is affecting tomato plants this summer. The New York Times reports it’s been wiping out tomato crops in the Northeast, and in the Hudson Valley region of New York, the disease has jumped to potato plants. This easily spread fungus is the same disease that caused the Irish Potato Famine in the mid-1800’s.

A very rainy June facilitated an early appearance for late blight in the Northeast. The damp July and cooler than normal temperatures has not helped. It will take “about 10 days with temperatures above 85 and dry conditions at night” to possibly stop the spread of the disease. Looking at the local forecast here in New Jersey, that isn’t happening any time soon.

The disease is affecting both farmers and individual gardeners. I’ve been checking my own tomato plants and paying close attention to every brown spot on every leaf. The University of Maryland’s Grow It Eat It website says to look for the following.

Lesions develop on leaves and stems as dark, water-soaked spots. These spots enlarge until the entire leaf or stem turns brown and dies. Dead leaves typically remain attached to stems. The undersides of the lesions may be covered with a white fuzzy growth that contains the spores of the pathogen. On the stems, late blight lesions appear brown to almost black. Infected tomato fruits develop shiny, dark or olive-colored lesions which may cover large areas. Potato leaves and stems will show the same symptoms. Infected potato tubers develop a dry, corky rot that often shows up in storage.