Late Blight on Tomatoes

Late blight of tomatoes, caused by Phytophthora infestans, is knocking on the doors of our state border. On Monday, July 6, we confirmed late blight in a commercial tomato field in North Georgia (Dillard, Georgia). Recent rain and cool temperatures have been conducive for the pathogen’s growth and spread, so we are concerned the disease will soon be occurring here.

Without proper preventative measures, late blight can completely defoliate and destroy a crop within one to two weeks. The disease can be severe on tomatoes grown in the mountains of North Carolina, as well as in late plantings in the Piedmont.

The first symptoms of late blight on tomato leaves are irregularly shaped, water-soaked lesions on lower leaves. During high humidity, white cottony growth may be visible on underside of the leaf. As the disease progresses, lesions enlarge causing leaves to brown, shrivel and die. Fruit lesions appear as dark, greasy spots that eventually turn a chocolate brown color, and can enlarge to the point of encompassing the entire fruit.

Refer to the following website for more details on the symptoms of this disease. The application of fungicides plays a significant role in the control of late blight. Fungicides containing copper, chlorothanonil, or mancozeb are recommended for treatment in home gardens.

New breeding lines resistant to some strains of P. infestans have recently been developed at the Mountain Horticultural Research and Extension Center in Fletcher, North Carolina by tomato breeder Dr. Randy Gardner. A new small fruited variety called Mountain Magic that has resistance to some strains of P. infestans, in addition to early blight, should be available to growers in the future.

Taken from Pest News - Volume 24, Number 13, July 10, 2009 - Kelly Ivors, Extension Plant Pathologist, NCSU