Ray Mueller
Correspondent
MADISON
On July 14, outbreaks of late blight were confirmed in a section of a Marquette County potato field and in tomatoes in Waukesha County. In 2009, the first confirmation of the fungal disease – on tomatoes – did not occur until July 29 in the state.
With the confirmation of the fungal disease in potatoes, Wisconsin joined Michigan and North Dakota as the third state on that list this year. Late blight had already been reported on tomatoes in Indiana, Minnesota, Ohio, New York, Kentucky, Maryland, Louisiana, Pennsylvania and Manitoba province in Canada by July 1.
Wet weather, which has been prevalent in much of Wisconsin since the second week of June, is the culprit for the devastating disease, according to Amanda Gevens, the University of Wisconsin – Madison plant pathologist who is monitoring the situation and providing advice to growers of tomatoes and potatoes. Regarding the outbreak in the potatoes, she indicated that the infection probably occurred five to seven days before the identification.
Often referred to as “water mold” because it thrives in wet conditions, late blight’s symptoms are leaf lesions that start as pale green or olive green that then become brown or black while being water-soaked or oily in their appearance, Gevens explained in a July 1 report that has since been updated on the http://www.plantpath.wisc.edu/wivegdis/ Web site. Other symptoms are white or gray fuzzy growths on the leaves and dark brown to black lesions on the stems.
On tomatoes, golden to chocolate brown lesions or spots – sunken with distinct rings – appear on the fruit, Gevens notes. Depending on the weather, the incubation period can be as fast as seven days, she adds.
With potatoes, the symptoms of late blight are dark brown to purple discoloration of the tubers, brown water-soaked and white pathogen sprulations on the leave, and brown water-soaked lesions on the stems, accompanied by a prolific outbreak of white pathogen spores.
Gevens points out that the strain of the disease isolated in 2009 from both tomatoes and potatoes was a new one and was particularly aggressive on tomatoes. Despite the number of outbreak incidents already recorded this year, she reports that the type of late blight has not yet been identified.
There is no cure for late blight once an infection has occurred, Gevnes emphasizes. “It is critical that tomato plants be protected before the first infection.”
Fungicides containing copper have been shown to be the most effective with late blight on both tomatoes and potatoes but they must have been applied before the first infection and must also be applied repeatedly, Gevens explains. “Copper products must be present on new foliage to have a protective, disease-slowing effect. So repeat sprayings are necessary.” For organic production, some of the copper products are not acceptable.
A list of product suppliers for both organic and standard production is included on the plant pathology web site. Growers who notice plants or fruits with probable symptoms are encouraged to contact a crop consultant, their county Extension Service horticulture or agriculture agent, the plant disease diagnostic laboratory in Madison, or Gevens by e-mail to Gevens@wisc.edu.
