The fungus that caused Ireland’s potato famine more than 150 years ago is starting to make its presence known in Wisconsin.

Thirteen counties in Wisconsin, not including Kenosha County, and several other states have confirmed cases of late blight, a fungus that attacks tomatoes and potatoes. Barb Larson, horticulture educator for the Kenosha County University of Wisconsin-Extension, said late blight was the cause of 19th century potato famines in Ireland and the fungus can be extremely destructive.

Amanda Gevens, plant pathologist for the University of Wisconsin and UW-Extension, said the fungus first was found in Dane County on July 29 and has spread rapidly around the state.

“In a month’s time it has had a great increase in area and severity and spread from tomatoes to potatoes,” Gevens said.

Kenosha County has not reported any official cases yet, but recent weather conditions have been prime for the spread of this fungus.

“We’re not sure where this came from, but the high dew points and cooler temperatures have made it perfect conditions for this fungus,” Larson said. “We have confirmed it on tomatoes in Walworth County and western Racine County, but we have not seen any definite ones in Kenosha County, but it’s a very strong possibility.”

The fungus, which has not been seen in Wisconsin since 2002, has seemed to mostly attack tomatoes in the state so far.

The first sign of late blight is gray-green spots on the center or tips of leaves of a tomato or potato plant.

“It’s described as water-soaked because those spots look like someone dropped water on the leaves,” Larson said.

In later stages, the spots grow and turn brown and nearly black as the fungus grows. Brown-and-black lesions then spread to the plant’s stem and, within a week or two, a whole plant becomes infected and the plant is reduced to hanging black leaves. The fruits of the plants also will end up with brown, sunken areas.

“It looks like someone burned the fruit on the top or the side,” Larson said.

Gardeners who find this fungus are encouraged to pull out the plant and destroy it, either by throwing it away with the regular garbage or letting it dry and burning it.

“The fungus spreads on plant matter, so we want to make sure that we destroy any plant matter that the fungus sits around on,” Larson said. “We don’t want to let it sit around.”

Kenosha residents also can bring plants they suspect of being infected to the Kenosha County UW-Extension office at the Kenosha County Center in Bristol at 19600 75th St. for further examination. Master gardeners are available at the extension on Monday, Wednesday and Friday mornings.

For gardeners looking to prevent late blight, Larson said copper fungicides can help, but people should make sure the fungicide is clearly marked for the plant they are looking to protect.

Larson said unaffected parts of tomatoes and potatoes probably are safe to eat, once the infected areas are cut out, but they may not taste good. Tomatoes affected by blight should not be canned or frozen at all.

Curzio Caravati, a local grower of potatoes and tomatoes and vice president of the Kenosha HarborMarket, said he has not seen any signs of the fungus in this area yet. But Caravati said potato farmers in the eastern part of the country have been dealing with late blight.

Wisconsin is a large producer of potatoes, and Larson said there are concerns about the spread of late blight.

“We have people who depend on tomatoes and potatoes, and we want to stop the spread if at all possible,” Larson said. “In Wisconsin, the strain that affects potatoes does not seem to be really strong, so we hope it does not have a significant economic impact. But it is very possible that it will have a tomato impact. It seems to be spreading faster and seems to be pretty aggressive.”

But Larson also said the massive damage caused by this fungus 150 years ago is not likely to be replicated.

“Are we worried about everyone starving to death and not having potatoes? No,” Larson said. “Our plant pathology is much more advanced. We know what to do and how to manage it.”

Residents with questions about the fungus can reach Larson at the UW-Extension at 857-1945.