Late blight, a plant disease that can kill tomato plants, has been
found on North Carolina tomatoes earlier in the growing season this
year than usual, according to a Cooperative Extension plant
pathologist at North Carolina State University.

Late blight was found several weeks ago on tomatoes in Northampton and
Sampson counties and on July 3 in Henderson County, said Dr. Kelly
Ivors, associate professor of plant pathology and North Carolina
Cooperative Extension specialist. Ivors speculated that the plant
disease showed up earlier this year because this spring was usually
warm.

While commercial tomato growers are generally aware of the presence of
late blight and are taking measures to combat the disease, Ivors said
home gardeners may also want to be aware of the disease.

Ivors pointed out that prevention, applying a fungicide or other
treatment to tomato plants before they are infected, is the best
course of action when it comes to late blight. Ivors said plant
protection products containing the active ingredients copper or
chlorothalonil offer the only effective protection for the home
gardener against late blight. Ready-to-use formulations of products
containing either of these active ingredients are available at garden
centers and stores such as Home Depot or Lowes.

While there are a few tomato varieties that are resistant to late
blight, Ivors added, heirloom tomatoes, which many home gardeners like
to grow, are not resistant to the disease.

Late blight, which also attacks potatoes, is caused by a fungus-like
organism called Phytophthora infestans. The pathogen is best known for
causing the devastating Irish potato famine of the 1840s, which killed
over a million people, and caused another million to leave the
country.

The pathogen likes cool, wet weather. Clouds protect the spores from
exposure to UV radiation, while wet conditions allow the spores to
infect when they land on leaves. Late blight can be a particular
problem in western North Carolina, where nights are cool and fog or
heavy dew can help spread the disease.

Ivors said lesions will appear on a plant’s leaves within three to
five days of infection, followed by a white cottony growth on the
underside of leaves. The cottony growth is evidence that the pathogen
in producing spores. Spores may be spread by wind and rain and can be
blown several miles, where they may land on other plants and start a
new cycle of infection. The disease eventually defoliates and kills
the plant.

Ivors said home gardeners who want to see the fruit on their tomato
plants turn ripe and red may want to consider protecting those plants
now. Once plants show signs of late blight, she added, the best option
may be to harvest the fruit, even if it’s green, and learn how to make
fried green tomatoes.

Written by: Dave Caldwell, College of Agriculture and Life Sciences
communications, 919-513-3127 or dave_caldwell@ncsu.edu