Late Blight Strikes Tomato and Potato Growers Statewide

By Dr. Vern Grubinger, University of Vermont Extension:
It's been an exceptionally rainy and cool summer and one of the consequences is that Late Blight disease has been reported in every corner of Vermont and across the Northeast. With these weather conditions Late Blight rapidly kills the foliage of tomato and potato plants. Many hundreds of farmers and gardeners have already been stricken, and it is likely that the situation will get worse unless the weather turns hot and dry.

Late Blight is caused by a fungus, Phytophthora infestans, and it's the same disease that led to the Irish potato famine almost 150 years ago. The disease is not directly harmful to people as it only infects potatoes, tomatoes, and some related weeds.

The good news is that the disease does not persist in the soil from year to year, so with proper action, farmers and gardeners should be able to avoid this problem next year. In addition, infected potato crops may still yield edible tubers if the diseased foliage is destroyed soon after infection is observed.

Late blight needs living plant tissue to survive, so infected tomato plants should be destroyed as soon as the disease is identified. In small gardens, this means removing plants in trash bags and sending them to the landfill; in larger gardens and farms the plants should be gathered into piles and burned, or simply turned into the soil so they can decompose. There is still plenty of warm weather that will allow plant residues to break down before winter. A month or so after initial incorporation, the residues should be lightly tilled and mixed with soil again, and then a winter cover crop should be sown.
Winter rye or oats planted in early fall will protect the soil over winter and further promote biological activity. Do not put plants in the compost pile just in case some portion is protected from the elements and makes it through the winter.

Once potatoes are confirmed with late blight, the tops (vines) should be mowed or cut off before the stems get heavily infected. That will help prevent spores from washing down to the tubers. Wait to dig the tubers at least 2 or 3 weeks until the vines are completely dead, as that will limit the number of spores on the soil surface when the tubers are dug. It also allows time for the tuber skins to toughen up underground, and that will limit the number of cuts and bruises created at harvest, reducing places for spores to get into the tubers.

When harvesting, be sure to get all the potatoes out of the ground, to limit the chances of the disease surviving the winter in a living tuber. After harvest, do not wash the tubers until it's necessary, as that could spread disease among them. Keep them in a cool location to suppress disease development, and check them often, removing any rotten tubers.

Late blight spores are easily carried long distances on the wind, so anyone growing tomatoes or potatoes should be watching their plants for signs of the disease, and act quickly to destroy them in order to limit spread of the disease to other growers. Please inspect your tomato and potato plants on a daily basis! Late Blight is already widespread and has caused serious economic losses. We need to do what we can to limit the scale of this epidemic.

The symptoms of late blight on tomato and potato leaves and stems and fruit are dark, water-soaked spots of irregular shape, about the size of a nickel to a quarter, often beginning at leaf tips or edges. These spots become abundant when the foliage has been wet due to rain or dews. The infected areas often become covered with light layer of white fungal growth that contains the spores of the disease. Infected tomato fruit develop large brown areas, either on the plant, or a few days after harvest.

There are other common diseases of tomato and potato that can be mistaken for late blight. If the infected area has a yellow border and is occurring on the bottom of the plant, it is probably due to Early Blight or Septoria Leaf Spot. These two diseases are found in home gardens and farms every year in the Northeast, but they are less likely to kill plants, and they don't spread long distances.

At this point, and with rainy weather, fungicides do not appear to be slowing the disease very much, but if your plants show no signs of infection so far, then fungicides may help to protect them. Homeowners can apply a garden fungicide labeled for tomato or potato use that contains the active ingredient chlorothalonil. Organic growers can apply a copper fungicide labeled for these crops. These products can only be effective if used before the disease appears and they should be reapplied every 5-7 days if wet weather persists. 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 many pictures of the symptoms, see:

Source: Dr. Vern Grubinger, Vegetable and Berry Specialist, University of Vermont Extension, July 30, 2009