Northeastern Indiana Gardens Infected with Late Blight Fungus

I was so proud of my garden this year. After writing so many gardening articles, I was ready to grow some tomatoes that made it look like I knew what I was talking about! I staked up my tomato plants to keep them off the ground, pinched off suckers, kept weeds under control, and fed the garden well. Many lovely blossoms became well-formed green tomatoes. Visions of BLT's with juicy red tomato slices danced in my head.

Unfortunately, my garden has fallen victim to a late blight that is spreading throughout home gardens in northeastern Indiana. The leaves of my tomato plants are turning yellow and the tomatoes begin to rot on the bottom as they ripen. The late blight is infecting vegetable crops in several counties in Indiana, including Wabash, Tippecanoe, Vermillion and Bartholomew. The infected plants were sold at major retailers (ours came from Wal-Mart) throughout the northeast Indiana region.

Home growers and commercial growers need to be aware of this highly destructive plant disease. The fungus can move from plant to plant and affect neighboring crops, and pathogen spores can be transferred on garden tools and farm equipment. Appropriate fungicides will prevent or limit the disease, but once exposed the plants will deteriorate quickly. Indiana's unusually cool, wet July created perfect conditions for this fungus epidemic to accelerate and spread.

How to Recognize Late Blight
Tomatoes and potatoes are the crops that are most susceptible to late blight, but the fungus can also infect other crops and ornamental plants. The disease can affect plants any time during the growing season, but the risk is highest during long wet spells.

The leaves of a plant infected with late blight will form small dark spots which enlarge in a bull's-eye pattern. The leaves may turn yellow and will have fuzzy white patches at the edges of the spots. The fruits or vegetables on the plants will form shallow brownish or purplish lesions on the surface which rot into the flesh. Late blight creates a pathway for other garden plant diseases to enter, and the signs may be difficult to identify when the weather isn't humid enough to create the fuzzy white spores.

How to Prevent Late Blight in Your Garden
There are no tomato cultivars that are resistant to late blight, although some are currently in development. You can buy certified disease-free and blight-resistant seed potatoes but there's no guarantee they won't become infected with late blight. Any garden plants that have spots on the leaves should be left on the retailer's shelf, and you'd be wise to take your business elsewhere if the retailer displays any obviously sick plants.

Use a preventative fungicide in your garden, especially when the weather has been unusually cool and damp. This step may not completely protect your plants from late blight, but could reduce the amount of damage it can cause. Follow the manufacturer's directions for safe use, and watch for signs of fungus infection throughout the season. At the first sign of infection, begin to use slightly more fungicide or apply it a bit more often to protect your crops.

Late blight can remain in potato tubers from one season to the next, and early "volunteer" plants may introduce the disease into your garden. Pull up any volunteer plants as soon as they emerge.

Pull up severely diseased plants and dispose of them in plastic bags immediately to prevent spreading the spores through the air. Doing this on a sunny, dry day with little wind is most effective. Never put sick plant matter in your compost pile.

Space your plants out so they won't be too crowded when they mature. Space between the leaves allows air to circulate and dry off excess moisture. Place your garden where it will receive early morning sunlight which will dry off the leaves.

Wind and rain can spread spores between plants, and the wind can carry the spores over long distances. Warn neighbors if your plants are infected with late blight so they can take preventative measures to protect their own gardens.

Resources:
"the paper" of Wabash County, Inc., p.3, August 12, 2009
http://www.nysipm.cornell.edu/publications/blight/
http://aperfectgarden.net/Tomato%20Diseases.htm