Preventing Late Blight in Gardens

Late blight caused by Phytophthora infestans – a very destructive and very infectious disease – killed tomato and potato plants in gardens and on commercial farms throughout the eastern U.S. during 2009. Late blight is the same disease that caused the Irish Potato Famine in the 1840s.  While late blight occurs at some locations in the Northeast each year, the occurrence of late blight in 2009 was different compared to most seasons. In 2009 infected plants were distributed through large local retail stores throughout the region (Ohio to Maine) during June, and outbreaks were reported over this entire region by early July. Never before had such an extensive distribution of infected plants occurred, especially so early in the season. The early introduction combined with the abnormally cool, wet growing season as well as the exceptionally contagious nature of the disease during cool, rainy, windy weather resulted in a disastrous year for farmers and gardeners. The following are some frequently asked questions about this disease and what home gardeners can do to prevent problems this year.

I think I had late blight on my tomatoes last year. Will it overwinter in my garden?
Tomatoes will not carry late blight over the winter, because freezing kills the whole plant.

Will seed from any tomatoes left in the garden last year carry over the disease?
Tomato seed, even from fruit that was infected with late blight, will not carry the pathogen, so no need to worry about the tomatoes left behind in the garden or compost pile.

What about my tomato stakes? Will it carry over on my tomato stakes?
No, late blight will not survive on tomato stakes and cages.

I grew potatoes last year. Can late blight overwinter on potatoes I may have left in the garden?
Potatoes that freeze or fully decompose will not carry the pathogen over winter. However, the biggest threat for overwintered disease in New England is on potatoes that might come up this year. In the spring, inspect last year’s potato plot and any compost or cull piles for volunteer potato plants. If you find potato plants, pull them out and put them in the trash or destroy them. If tubers were infected and survive, then the late blight pathogen could grow upward from the tuber, infecting the stem and producing spores when weather conditions are favorable. These spores could then disperse to other tomato and potato plants.

Is there anything else I can do to prevent late blight this year?
Yes. Late blight is not seedborne (however, it is tuber-borne in potato), so tomato plants started from seed locally (in the Northeast) will be free of the disease. Growing your own tomato transplants from seed or purchasing transplants that were started here in the Northeast from seed will ensure a healthy start to the season for you and local farms. Disease-resistant or tolerant varieties of tomatoes exist, however seed is in limited supply this year, so you will not likely find them at your garden center. ‘Mountain Magic,’ ‘Plum Regal,’ and ‘Legend’ are three varieties with resistance or tolerance to late blight. Note that the variety ‘Legend’ is the only late-blight resistant variety for which seed is readily available this year.

Also, purchase certified, disease-free seed potato from a reputable source. Seed potato grown in the Northeast is less likely to carry the disease. Examine your seed potatoes and plant only firm, blemish-free tubers.

I have spots on my tomato plants that show up in June or July every year. Could this be late blight?
Probably not. In addition to late blight, each year tomatoes become infected with early blight and Septoria leaf spot, which look very similar to late blight symptoms. If you have problems with early blight each year, consider growing tomato plant varieties that are resistant or tolerant to early blight, such as the varieties ‘Mountain Fresh,’ ‘Mountain Supreme,’ ‘Plum Dandy’ and others.

What does late blight look like?
Click here and here for photos of the disease on leaves, stems and fruit.

During the growing season, check your garden for symptoms of late blight regularly – at least weekly. Monitor more often during periods of wet weather. When disease first appears, begin treatment with a fungicide containing one the following ingredients: chlorothalonil, copper sulfate or copper hydroxide. These will also help to protect the plants from other foliar diseases.

Continued fungicide coverage will be necessary to protect plants from infection.

For more information about late blight and for pest alerts during the growing season, visit, www.umassvegetable.org

Tina Smith, Ruth Hazzard, M. Bess Dicklow
University of Massachusetts Agriculture and Landscape Program