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Gardening Columns

It's Not Too Early for Late Blight on Tomatoes

First published on on July 16, 2009, by Suzanne DeJohn

If your tomatoes are struggling this year, you're not alone. Late blight, the disease that caused the Irish potato famine in the mid-1800s, is affecting tomato plants across the Northeast. The late blight fungus, Phytophthora infestans, isn't uncommon in the tomato patch; what's uncommon is how early it's appearing this year and how much damage it's causing in both home gardens and on farms. What's also uncommon is that scientists have found the fungus on transplants for sale at various garden centers. That means that your garden plants may have been infected before you brought them home, and indeed researchers have identified at least one large-scale grower in Alabama as one possible source of infected plants.

Signs and Symptoms
If you've grown tomatoes you know that they're susceptible to a variety of foliar diseases. By the end of the season as the last fruits are ripening, tomato plants in even the best-kept gardens look scraggly with their foliage mottled and yellowing from diseases like early blight and septoria leaf spot. What's significant about this outbreak is that late blight doesn't just render plants unsightly, it kills them outright. 

For help in identifying late blight, I'll turn to Cornell University's Chronicle Online:

"One of the most visible early symptoms of the disease is brown spots (lesions) on stems. They begin small and firm, then quickly enlarge, with white fungal growth developing under moist conditions that leads to a soft rot collapsing the stem. Classic symptoms are large (at least nickel-sized) olive-green to brown spots on leaves with slightly fuzzy white fungal growth on the underside when conditions have been humid (early morning or after rain). Sometimes the border of the spot is yellow or has a water-soaked appearance. Spots begin tiny, irregularly shaped and brown. Firm, brown spots develop on tomato fruit."

Home gardeners and farmers alike are being encouraged to learn to identify the symptoms of late blight and to remove and destroy plants at the first signs of the disease to help limit its spread. This year's cool, rainy weather provides optimum conditions for the spread of many fungal diseases, and late blight is no exception.

If you purchased plants from a garden center, it might be wise to begin a preventative program now, as the disease is difficult if not impossible to control once it takes hold. If you grew your own plants from seed, keep a close eye on them as they could quickly be infected by spores carried on the wind. Some gardeners report success using a spray made from compost tea as a fungicide; there are several new organic fungicides on the market now, too.

For photos of tomatoes infected with late blight, visit this Cornell Coopertive Extension Web page:


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