(AP) — Crop diseases are at an all-time high in Minnesota.

“There are more diseases out there right now,” said Dean Malvick, University of Minnesota Extension specialist for diseases of field crops. “Because old ones have not gone away, and are still there, but we have new ones too.”

He said newer diseases might not become major problems for growers, but some of them have the potential to be.

Corn

Tar spot of corn is a disease that Malvick said was first found in the U.S. in 2015, and only seen in Latin America before then. Corn at the growth stage is susceptible to infection.

So how did it get here, if it was only seen in Latin America before 2015? Nobody really knows, said Malvick.

“It was first found in areas in Indiana and Illinois, and since then it’s been spreading,” said Malvick. “It’s a pretty significant disease of corn.”

Tar spot is caused by the fungus Phyllachora maydis, said Malvick, and produces “small, semi-circular raised black structures.” Tan regions may surround the black that are refereed to as “fisheye lesions.”

In 2018 it spread into Wisconsin and Michigan; it reached Minnesota in 2019. The disease was confirmed in a handful of southern Minnesota counties in the past year.

It caused yield losses estimating up to 30 bushels an acre, said Malvick of areas in Wisconsin, Illinois, Indiana and Michigan.

“It’s something we have to watch for,” he said. “Because it’s here, and it’s spreading.”

In a presentation last week, Malvick went over the ways to scout for tar spot and some of the challenges that come with the disease. For example, symptoms may be confused for another disease, or symptoms may begin with low severity but then increase rapidly.

Rotating to other crops, managing residue and considering fungicides are also suggestions he has for management if the disease is spotted.

Another disease that Malvick covered is bacterial leaf streak of corn, which is new to the state within the last five years. He said the disease is increasing in the state, and trials done by extension in Waseca showed that it can cause yield loss in some hybrids.

Southern corn rust, which Malvick said was also an uncommon disease for the state, was considered a “tropical disease” before it recently became more common in the upper Midwest. He said the disease is favored by warm temperatures and high humidity, and “infection requires 6-hour leaf wetness”.

Soybeans

Frogeye leaf spot is another disease that was fairly uncommon in Minnesota before 2018-19, but had reported yield losses up to 30 percent in southern states. Malvick said the disease “reached levels of concern in a few fields” this past year in Minnesota.

He said not only is the disease becoming more prevalent, but he knows at least one field in southern Minnesota that was sprayed with fungicides to manage frogeye leaf spot.

“It’s the first time in Minnesota that’s ever happened,” he said.

Counties with confirmed cases of frogeye leaf spot in 2019 were Faribault, Dakota and Watonwan.