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Close to home with Mary Beth Breckenridge

Save the tomatoes!

By Mary Beth Breckenridge Published: August 15, 2014
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Late blight on greenhouse tomatoes.

Bad news for all of you who are patiently waiting for your tomatoes to ripen: Late blight is back.

The fungal disease can hit this time of year when the weather turns cool and either wet or very humid. And guess what we’ve had lately? Cool temperatures and wet conditions.

Ohio State University Extension’s Erik Draper found the fungus, Phytophtora infestans, on tomatoes in western Trumbull County, he reported in this week’s Buckeye Yard & Garden Line e-newsletter. He’s concerned that Northeast Ohio is poised for an outbreak.

If you want to save your tomatoes -- or your potatoes, which are also vulnerable -- now’s the time to treat them with a fungicide containing chlorothalanil or copper. Chlorothalanil is more effective against late blight, but if you grow organically, copper is your only option.

You shouldn’t have any trouble finding either type of fungicide at a garden center or home center.

Fungicides work only as a preventive measure. They won’t do any good if your plants are already infected.

And they’re not miracle workers. If the conditions are right, even fungicides won’t stave off the disease, OSU plant-disease specialist Sally Miller warned in the newsletter.

What does late blight look like?

It first appears as pale green, water-soaked spots on leaves, often beginning at the tips or edges. The spots can be circular or irregular and are often surrounded by a pale, yellowish-green border. The spots enlarge rapidly and turn dark brown to purplish-black. 

In wet or humid weather, a cottony white mold usually starts growing soon after the spots appear.

If your plants become infected, pull them out of the ground, put them in a plastic bag, close the bag with a knot and throw it away. Don’t compost diseased plants or leave the plants or debris in your garden.

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