Late blight, a disease that once affected primarily late-season potatoes, has turned into an irritant that now can rear its head as early as May. And the disease has significantly increased its impact on commercially grown tomatoes.

Known to the scientific community as Phytophthora infestans , late blight was responsible for the seven-year Irish Potato Famine that started in 1845.

In the United States, it threatened tomatoes and, to a lesser extent, potatoes in 26 states from early spring through August 2009. It was most prevalent in the Midwest and on the Eastern Seaboard.

Its source was traced to tomato transplants sold at big box stores.

By late June of this year, it had shown up in Connecticut, Kentucky, Louisiana, Maryland, Michigan, New York, North Dakota, Ohio, Pennsylvania and in Southern Manitoba, Canada, says Amanda Gevens, assistant professor of plant pathology at the University of Wisconsin in Madison.

There also were reports that the disease was found in at least one field in California.

Researchers expect late blight to make an impact again this year, because residual conditions, such as infected plants that over-winter, typically spill over into the next year, Gevens says. However, its impact tends to lessen from year to year.

Last year’s strain favored tomatoes over potatoes.

“We’re seeing that pattern again this season,” Gevens says.

The Midwest experienced a lot of rain and extended periods of leaf wetness this year, she says, adding, “Under those conditions, the late blight pathogens thrive.”

Late blight predominantly attacks tomatoes and potatoes, and it is a weaker pathogen on other solanaceous plants, such as peppers and eggplant, she says.

Early detection

Late blight was confirmed in potatoes in North Dakota, June 15, the earliest it ever was reported there, says Neil Gudmestad, professor of plant pathology and university distinguished professor at North Dakota State University in Fargo. The previous record was June 29, 1994.

For the first time in years, the disease was reported in California, an area where late blight is relatively rare because of its arid climate. Gudmestad theorizes that the disease may have come in on seed from the East Coast.

Late blight also was found early this season on tomatoes in Michigan, where he says it’s a chronic problem because of the proximity to the lakes, the moist environment, and the presence of both the tomato and potato industries.

“It is unusual when they don’t have late blight on either crop [in Michigan],” he says.

The disease also is relatively common in Maine.

“Second-tier” states where it may show up include New York, Florida and the Pacific Northwest, Gudmestad says.

What to look for

How do you know if your crop is suffering from late blight?

“Initially, you’ll see a water soaking of foliage,” Gevens says.

The leaves or stems of the tomato or potato plant look wet, they are a darker green or darker brown color, and they look oily or slick.

They’ll quickly turn from wet to “just being dead,” she says.

Late blight also can infect the tomato fruit. On green fruit, the disease manifests itself in the form of a hard or firm lesion that is brown in color. You’ll often see concentric rings as the pathogen grows.

When conditions are right, you can easily see the pathogen producing new spores on the infected plant in the form of a fuzzy, white growth that looks like white powder, Gevens says.

A plant may survive a light infection of late blight with the aid of fungicides if the weather is hot and dry. But if the infestation is heavy, there’s no reversing it, she says.

Easily preventable

The good news is that late blight generally is relatively easy to prevent with common fungicides that you already may apply.

Late blight struck Red Gold Inc., an Ellwood, Ind.-based tomato processor, last year. But the company will take steps this season to keep its tomatoes late-blight free, says Steve Smith, director of agriculture.

Growers for the company, which sources 80 percent of its tomatoes from Michigan, found late blight in areas around light poles and at the end of the fields where sprayers weren’t up to full capacity.

The company also used a new product that did not offer good late-blight control last year.

“This year, we have plugged those holes,” Smith says.

Red Gold isn’t letting its guard down, though.

“Given the weather we’ve had and that [late blight] is showing up all around us, we’re being very vigilant,” Smith says.

The company’s standard spray program includes Quadris, Tanos and Bravo. And this year, the firm is adding Revus Top to replace the spray that did not offer good late-blight protection.

The standard spray regimen should protect against late blight. But if a crop experiences a massive outbreak, growers can add Presidio and Ranman and increase applications of Revus Top, Smith says.

Late-blight-specific applications can cost $50 to $80 per acre, he estimates.

In some ways, fresh-market growers can be more susceptible to late blight than processors. Small, independent farmers may not have the same networking capabilities as larger processors, who can monitor the progress of the disease and recommend preventive actions, Smith says.

Preventative measures

“Controlling late blight on tomatoes is easier than you think,” says Mary Hausbeck, an East Lansing-based professor and Extension specialist at Michigan State University, who wrote an article stating just that. The article is available at

The key is controlling the disease on potatoes, she says.

“Control measures include eliminating all potato and/or tomato cull piles and destroying volunteer potato plants that grow from overwintered tubers,” she says in the article. “Infected potato plants established from diseased seed potatoes are another source of late blight.”

An eventual solution may be late-blight-resistant tomato varieties, such as the Mountain Magic recently developed at North Carolina State University. However, that variety is in short supply, Hausbeck says.

A resistant tomato or potato variety may someday be an effective alternative, Gevens agrees.

“It’s a piece of the puzzle, and it’s continually being developed, but it’s certainly not the answer for this season,” she says.

Meanwhile, researchers advise growers to be alert for late blight outbreaks this year.

“We are set up in 2010 in many places in the United Sates to have late blight that could be as serious as it was in 2009,” Gudmestad says.

“I don’t like the position we’re in. It makes me feel uncomfortable.”