A historically significant plant disease has been found in Marinette County and is potentially present throughout the entire area. Late blight, the disease that caused the Irish potato famine, was confirmed to be the causal agent of disease symptoms found in a home tomato garden.
Late blight is a devastating disease of potato and tomato that might also infect pepper and eggplant. In cool, wet conditions the disease can cause plant collapse and death in as little as seven days. The strain of late blight present is known for its virulence to tomatoes and its very high spore production.
All home gardeners and commercial vegetable producers need to monitor their plants for this disease. Key symptoms to look for are the speed of disease progression, stem lesions, and the fruit or tuber lesions. The leaf symptoms can be somewhat similar to two very common diseases that gardeners may experience every year, early blight and Septoria leaf spot. However, the leaf lesions quickly enlarge to black, water-soaked areas that are almost oily looking.
Stem lesions are not at all common with the other tomato and potato diseases, but will occur with late blight. The stem lesions will be dark gray to black and can form on both younger and older stems.
Tomato fruits that are infected will also have tell-tale symptoms. The fruits will develop large, usually sunken but still firm lesions that are golden to chocolate brown, having relatively distinct rings.
Potato tubers that are initially infected will have a reddish-brown discoloration under the skin and will usually have sunken tissue in the infected areas. Tubers will decay, and if put into storage, can infect the entire stored lot of tubers and turn them into stinking, mushy masses.
In addition, especially on wet mornings, you may be able to find white, fuzzy growth on any infected tissue. This is the spore-forming bodies of the disease pathogen.
The one positive of the Marinette County situation is the lateness of the disease’s arrival, because the treatment of infected plants is disposal of the entire plants. The recommendation for gardeners is to harvest any noninfected fruit or tubers, as they can be safely eaten, and then pull the plants and place them in plastic bags. Place the bags in full sun for a few days, and then send them to the landfill.
Do not compost diseased plant material or leave it in the garden. Any material left in the garden has the potential to allow the disease organism to survive over the winter. Commercial operations can contact the University of Wisconsin-Extension or other resources for assistance in managing the disease on their operations.
Fruit or tubers that have lesions on them should not be eaten or preserved because the presence of the disease causes the pH of the plant material to rise, increasing the opportunity for other pathogens to become present. The pH rise is also why tomatoes with lesions should not be canned, as it can create unsafe canning conditions.
Noninfected fruits can be eaten, but should be used quickly, as they may still develop lesions.
If in doubt, get additional information by contacting Scott Reuss, agriculture and horticulture UW-Extension agent, at 715-732-7510 or scott.reuss@ces.uwex.edu or find late blight publications on the Internet.
The Marinette County UW-Extension website, marinette.uwex.edu, has a late blight tab that connects to resources.
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