Late Blight
Martha Maletta, Home Horticultural Consultant
Dated - September 30, 2004

Late BlightI’ve finally seen late blight, a common and important disease of potato and tomato, in person. It was reported in New Jersey this year, and a County resident recently brought in a classic example of the damage on tomato. In my business, it’s always interesting to see a “new” problem, but this is one that can be devastating for commercial growers and home gardeners.

Fortunately late blight does not usually occur annually or extensively in New Jersey because it is a disease of wet and cool weather. It is a common problem wherever potato and tomato are grown, average temperatures are in the 50’s at night and 70’s during the day and wet conditions prevail. Certain areas of Pennsylvania seem to be hit with some regularity. Late blight, caused by the fungus-like organism Phytophthora infestans, is the disease that caused the disastrous Irish potato famine in the 1840’s.

The weather conditions we had at times this summer and particularly during the remains of recent hurricanes were perfect for late bight. In fact, the gardener who submitted the sample to RCE said that the plants were infected and died shortly after hurricane Ivan. Late blight infected tomato or potato leaves develop water-soaked spots and areas that turn pale green and then brown to purplish, even black. Under moist conditions white, cottony growth can be seen on the undersides of leaves at the edge of the spots. Stems may show symptoms, too. Plants can be killed in just a few days when conditions are right for disease development and spread.

Infected green tomato fruit has large dark brown-black but firm lesions. Potato tubers damaged by late blight have brown to purple, dry and sunken areas on the skin with coppery brown discoloration in the flesh under the skin. The damage may not be apparent at harvest but only after a period of storage.

RCE vegetable pathology specialist, Dr. Andy Wyenandt, tells me that home gardeners who had or suspect they had late bight in the garden should remove all plant debris from the garden or till it under very well. Tomatoes and potatoes should be rotated and not follow each other in the garden. It had been thought that late blight did not survive winter in New Jersey, but the experts are not so sure about that now. During the growing season, a plant suspected of having late blight should be removed immediately and destroyed.

Most references indicate that any tomato or potato plants that volunteer the next season (grow from this year’s remains) should be destroyed immediately. Spraying a fungicide to protect plants before infection occurs may be helpful, but there are strains of late blight, present in recent years, that are particularly difficult to control.

A relatively new disease is causing serious concern in the northwest and in California where it was identified in 2000. Sudden oak death, caused by another Phytophthora species, ramorum, causes a bark canker that kills several western tree species. It also causes foliar disease on many plants including some that we commonly grow here: rhododendron, viburnum, andromeda, lilac, and mountain laurel.

The USDA has moved aggressively to restrict movement of host plants from the affected areas and inspect nurseries in many states. Earlier this year it was discovered that some potential host plants from had been shipped to New Jersey from California. The NJ Department of Agriculture and the USDA Animal and Plant Health Inspection Service (USDA-APHIS) cooperated to find and test suspect plants and educate the nursery industry and the public. The USDA has published a National Pest Alert Fact Sheet on this disease. For a copy of the Fact Sheet and links to more information on sudden oak death, a disease that, with all the effort to contain it, will, hopefully, never become established in New Jersey, visit