Corn, soybean disease won’t be hard to spot in 2020

Tar spot is on Extension’s most wanted list for corn crop growers this season. It can be easily identified by the tar-like black spots on both sides of corn leaves. Phyllachora maydis and/or Monographella maydis in Latin America crop. Strain was confirmed in the U.S. in 2015. Symptoms include: black dots(ascomata), “Fisheye” rings, and <50% yield loss.

Disease pressure in corn and soybeans had a significant impact on Nebraska producers last year. Experts are saying that 2020 appears to be lining up for substantial disease influence.

Crop specialists and other professionals are informing farmers to be primarily alert for the presence of eight particular pathogens, six which affect corn and two for soybeans.

Corn growers are to be watchful for tar spot, bacterial leaf streak, physoderma brown spot, grey leaf spot, rust and ear rot diseases. Tar spot is of substantial concern.

“Tar spot has been expanding rapidly in the last two years,” said Kevin Scholl, a crop specialist with Syngenta.

It takes hold in cooler and wet areas. Places with high relative humidity (greater than 75%) and temperatures around 60-70 degrees favor tar spot development, he said. Fog or heavy dew increase the chances.

“It has shown it can overwinter,” Scholl said. “Only a little bit can go a long way.”

According to the University of Nebraska-Lincoln Extension information, tar spot is caused by the fungus Phyllachora maydis. The fungus produces small (0.2-0.8 inch), round to semi-circular raised black structures called stromata. The structures form on both the upper and lower surfaces of corn leaves.

“There is a latent period,” Scholl said. “The time from infection to visibility is from 14 to 40 days.”

You can diagnose corn tar spot in the field by examining corn leaves for the presence of black, tar-like spots. To date, tar spot has been observed most often during growth stages R3-R6 and usually on leaves below or near the ear leaf, said Dr. Tamra Jackson-Ziems, UNL Extension plant pathologist.

Those raising soybeans this season should be on the lookout for target spot and frogeye leaf spot. The biggest concern centers on target spot.

“Target spot is moving up from the south fairly quickly,” said Phil Krieg of Syngenta. “Primarily it affects broadleaf plants. It also affects tomatoes.”

According to Extension information, target spot is a foliar disease that has been reported in all soybean growing regions of the U.S. Yield losses of 18% to 32% have been reported.

Leaf lesions are reddish-brown round to irregularly-shaped spots that range in size from three-eighths to five-eighths of an inch in diameter. Lesions are frequently surrounded by a yellowish green halo. Infected areas on stems and petiole are dark brown and range from specks to elongated lesions. Lesions on pods are typically small (1/32 of an inch), circular purple or black spots with brown margins, said Dr. Jackson-Ziems.

Both tar spot and target spot are very environmentally-driven diseases, Krieg said. Even crop rotation won’t work to manage as spores can travel from neighboring fields.

“Usually neither of these would be the only pathogens attacking fields,” he said. “So fungicide could be used on crops.”

He suggests using a fungicide with long residual to get to R1 stage for soybeans. But early treatment is crucial. Even as early as V5 or V6, he said. He suggests something like Trivapro or Miravis Top.

“The thing that has really changed over the past few years is the appearance of new diseases,” Krieg said. “There is a lot more residue (for erosion control) in the top soil. It creates the climate for these diseases.”

Should you suspect tar spot or another pathogen, send samples to the UNL Plant and Pest Diagnostic Clinic. For more information about the clinic, visit

Jon Burleson can be reached at

Jon Burleson is the Midwest Messenger reporter, based out of eastern Nebraska. Reach him at