Extension issues tomato blight warning
UW plant pathologists advise commercial growers and home gardeners to begin fungicide treatments to help prevent late blight in tomatoes and potatoes according to Mike Ballweg, UW-Extension agricultural agent for Sheboygan County.
Late blight, Ballweg said in a press release, is a devastating plant disease that wiped out much of the tomato crop in eastern Wisconsin in 2009. It is the same plant disease that caused the great Irish famine that destroyed Ireland potato crops in the 1800s.
While the disease has not yet been reported in Wisconsin, there are confirmed cases in the Upper Peninsula of Michigan and Benton Harbor in the southwestern corner of Michigan. These are in close enough proximity to pose a threat to Wisconsin. Late blight reports have emerged from Maryland, Pennsylvania, Kentucky, southern Manitoba, Canada, and most recently New York and Connecticut. Thus far, all have been on tomatoes.
Sprays containing either copper for organic gardeners or products with the active ingredient chlorothalonil are effective at preventing this devastating disease. Treatment must begin before infection occurs and applications need to be repeated as new leaves develop throughout the season, usually every five to 10 days depending on the weather and fungicide labels.
Late blight disease is more active during cool, wet weather and progresses more slowly when it is hot and dry. Under ideal conditions, infected plants will die in seven to 10 days. Spores move on the wind spreading the disease up to 40 miles at a time and leapfrogging to cover huge areas quickly. Fungicide products are readily available at most local retailers that carry garden pesticides.
Late blight should not be confused with other common diseases of tomatoes. Late blight causes pale or olive green blotches on the leaves that quickly turn brown-black, water-soaked and oily looking. Dark brown to black patches can also form along the stems.
On the tomatoes themselves, the disease causes large, sunken, golden- to chocolate-brown, firm spots with distinct rings. A grayish fuzz can form eventually on leaves, stems and tomatoes.
The other two common diseases of tomato, septoria and early blight, have either small, circular spots with tan centers or larger brown spots with a bulls-eye pattern. These begin on the lower leaves, turning the leaves yellow at first and then brown and dry. They do not affect the stems and do not spread rapidly, seldom killing the plant. Fortunately, the fungicides recommended to treat late blight are also effective in treating septoria and early blight.
If you have the misfortune of developing late blight on plants in your garden this year, do not compost any parts of the plants. Instead, pull out plants roots and all, bag them in black plastic and leave them in the sun for several days before disposing of them in the trash. Remove all infected potatoes from the ground and treat similarly. Growers can shallowly turn under infected plants so that all spores are killed by freezing this coming winter.
For further information visit the Sheboygan County UW- Extension Web site at http://sheboygan.uwex.edu. Master Gardeners are available Tuesday and Thursday afternoons at the Sheboygan County UW-Extension office to assist homeowners with garden and yard related questions. They can be reached at 459-5900.
