If you grow tomatoes or potatoes, take heed. This is shaping up to be one of the worst years for Late Blight, the fungal disease made famous as the cause of the Irish potato famine of the mid-1800's. Today, this disease doesn't pose the threat of famine, since we don't depend on one or two local crops for our food, but it could ruin potato and tomato crops for local farmers and gardeners unless they are alert and take appropriate action. Note that this disease poses no threat to people -- except for the loss of these crops.

Late blight is caused by the fungus called Phytophthora infestans, and it's actually not uncommon in the Northeast, since it thrives in cool summer temperatures and frequent rains. But usually its occurrence is limited to later in the growing season and only certain areas of the region, typically in a few farm fields. This year, it has shown up early and is widespread. Worse, it's been identified on tomato plants for sale at a number of home garden centers, suggesting that large numbers of home gardeners have purchased infected plants, which may serve as a source of inoculum (spores) that can spread the disease.

Late blight inoculum is easily carried long distances by wind currents, so anyone growing tomatoes or potatoes should be on the lookout for signs of the disease, even in the most remote areas in our region. Currently all varieties of tomato and potato plants grown in home gardens and in commercial fields are susceptible to late blight. If your plants have late blight, be prepared to destroy them in order to limit spread of the disease.

The Late Blight organism is not seedborne (however, it is tuberborne in potato), so that tomato plants started from seed locally should be free of the disease, at least initially.

The symptoms that develop on tomato leaves, stems and fruit are quite dramatic, and are very obvious to the naked eye. The infected areas on leaves appear to be water-soaked, varying in size from a nickel up to a quarter, often beginning at leaf tips or edges. They proliferate when the foliage has been exposed to watering, rainfall, or heavy overnight dews. If these infected areas dry out quickly, they may appear lime-green or beige in color.

The edge of the water-soaked area, either on the top or bottom of the leaf surface, will be covered with white fungal growth (mycelium) that contains the spore inoculum (visible with a hand lens). Spores are easily blown to surrounding areas and infect plants and even weed species, in the plant family Solanaceae (the black nightshade family).

Brown to almost black lesions appear on infected stems, and the same lesions will develop on fruit, either directly on the infected plants, or a few days after they are sitting on your kitchen counters.

Be aware that there are several other common, but less serious, diseases of tomato and potato. If the infected area has a yellow border and is occurring on the bottom of the plant, it is likely due to either Early Blight or Septoria Leaf Spot. These two diseases are found in home gardens most every year in the Northeast, but they rarely kill the plants, and they don't spread long distances.

Please inspect your tomato and potato plants on a daily basis! If Late Blight symptoms are already appearing on plants in your garden, these plants should be removed immediately and put in a plastic bag for disposal. Don't just put the removed plants in a compost pile as spores will still spread from this debris. Your neighbors, not to mention commercial growers, will appreciate your taking this action immediately.

Commercial growers have a number of fungicides that if applied early and on a regular basis, can reduce the spread of Late Blight. They would choose not to spray if they could, but this destructive disease does not give them any other option. Homeowners do have a few products that are registered for use and the common name of chlorothalonil should appear on the product label. Even here, these products are only effective if used before the disease appears and should be reapplied every 5-7 days if wet weather persists. For organic growers the options are very limited. Copper fungicides can be used, but they are not very effective.

If spraying any type of fungicide, remember that these materials only protect healthy tissue -- infected leaves cannot be saved. Good coverage of all the foliage is critical, and repeat applications are needed to protect new growth from infection. Always read the pesticide label and follow the instructions carefully.

For more information on Late Blight, including pictures of the symptoms, see:

http://vegetablemdonline.ppath.cornell.edu/factsheets/Potato_LateBlt.htm
http://ohioline.osu.edu/hyg-fact/3000/3102.html

Dr. Vern Grubinger is a Vegetable and Berry Specialist with the University of Vermont Extension.