EAST LANSING, Mich. — The first late blight incidence of the season in Michigan was confirmed July 15 in potatoes near Edmore, Mich., in Montcalm County.
Dr. Willie Kirk, extension plant pathologist at Michigan State University (MSU) said this is the only reported occurrence so far in Michigan, but it was found July 6 on potatoes in northern Indiana, in a field about five miles from the Michigan border.
Late blight is one of the most devastating diseases of potatoes and tomatoes worldwide. Since 1990, late blight has caused widespread damage throughout the U.S. and Canada. The disease, which caused the Irish Potato Famine in 1845, can result in complete destruction of potato or tomato crops if left unmanaged.
Kirk said this year’s onset of the disease was earlier than usual. Last year, late blight was not detected until October, when it was confirmed in a tuber sample. The disease normally does not show up in the crop until the end of July or later in the summer.
"It was found in a part of the field that probably didn’t get well covered with fungicide," Kirk said. He explained that aerial application of fungicide is sometimes less effective on the edges of fields. "It probably only got 20 to 50 percent of the coverage needed for protection."
Since the discovery, the infected crop was destroyed. However, Kirk said it is likely that the field contains more infected plants. The field is being scouted daily for signs of additional disease.
"The grower is applying fungicide now using a ground rig, for better coverage," he said.
Kirk said recommendations for control of the disease call for spraying every five days at this time.
The first symptoms of late blight are small, dark, circular to irregularly shaped lesions that appear on the leaves three to five days after they are infected. These lesions spread rapidly in cool, moist weather into brown to black spots that are often surrounded by a green border. The disease is spread from infected plants in one area to another by wind, splashed rain, animals and mechanical transport such as equipment.
According to Kirk, the pathogen favors wet weather with moderate temperatures between 60 and 80 degrees Fahrenheit, high humidity and frequent rainfall.
"Under such conditions, the disease can spread extremely rapidly and has the potential to completely defoliate fields within three weeks of the first visible infections if no control measures are taken," Kirk said.
In addition to attacking foliage, the disease can infect tubers at any stage of development before or after harvest, and soft rot of tubers often occurs in storage following tuber infections.
Kirk said cooler weather conditions across the area "is the least likely weather for the spread of late blight. We’re lucky that we have a few days of good weather."
Michigan growers can assess their risk and get fungicide recommendations at www.lateblight.org, a site that uses weather station data from the Michigan Agricultural Weather Network to calculate disease severity values. In addition, growers from all parts of the eastern U.S. can benefit from www.usa blight.org, a site which tracks disease outbreaks across the eastern U.S.
Michigan ranks No. 8 in potato production, according to the USDA National Agricultural Statistics Service, and Montcalm is the state’s largest potato-producing county.
