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Prevent Late Blight Now

Row of tomato plants dying from late blightTreatments to prevent late blight on potatoes and tomatoes should begin now, according to UW plant disease specialists. Untreated, infected plants in home gardens or farm fields pose a serious threat to the state’s commercial potato and tomato growers who could experience huge economic losses this season.  According to Sharon Morrisey, Consumer Horticulture Agent for the Milwaukee County UW-Extension, “this is what is considered a community disease since everyone needs to control it for us to really have it under control.”

Sprays containing either copper for organic gardeners or products with the active ingredient chlorothalonil are both 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. 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 7 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. “The good news is that these products are readily available at most local retailers that carry garden pesticides”, Morrisey said.

While the disease has not yet been reported in Wisconsin, there are confirmed cases in Michigan in the UP and Benton Harbor in the southwestern corner of the state.  These are in close enough proximity to pose a threat to WI.  Throughout the season already late blight reports have emerged from LA, MD. PA. KY, southern Manitoba, Canada and most recently NY and CT.  Thus far, all have been on tomatoes.

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.

In 2009, late blight was reported on tomatoes in 26 WI counties and on potato in 5 but in reality it was probably more widespread. During the winter, more samples of infected potatoes were submitted to the UW Plant Disease Diagnostic Lab from potatoes being stored mostly by homeowners from last year’s crop. The fungal-like organism that causes the disease, Phytophthora infestans, can only survive the winter in Wisconsin on living plant tissue such as potatoes in storage or left in the field and infected tomatoes put in compost piles that were not managed properly. This compost should be destroyed and not used in the garden.

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.

Morrisey further commented, “People are pretty serious about their homegrown tomatoes.  There is nothing quite as good as a ripe tomato fresh from the garden.  Additionally, there are still a lot of people who make and can homemade pasta sauce, juice and salsa.  A crop failure can disappoint a lot more than just the gardener.”

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