DEKALB, Ill. — Farmers in northern Illinois saw lots of tar spot on their corn plants during the 2018 growing season.

“It was a tar spot explosion and from July 9 on with just a little effort you could find tar spot,” said Russ Higgins, University of Illinois Extension commercial agriculture educator. “One of my greatest surprises was at the end of September when I walked into a cornfield and didn’t find tar spot.”

The difference in 2018 compared to previous years is the disease showed up much earlier, Higgins said during a presentation at the Northern Illinois Farm Show.

“It would usually show up sometime in September, there would be a few lesions and it really wasn’t an issue,” he said.

“But in 2018 it was much more prevalent and then we found the eye spot on some of the tar spot lesions,” he said. “We have a lot to learn about this disease.”

The question is if tar spot will be a problem for the 2019 growing season.

“One thing we know is the potential to have a lot of pathogen around is relatively high since a lot of corn was infected this past year,” Higgins said.

“Last year at one of our variety testing locations southwest of DeKalb, every variety was affected by tar spot and we rated every hybrid,” he said. “The hybrids ranged from 24 to 44 percent infected and the yield loss was from 0.4 to 1 bushel for every percent of tar spot so the hybrid at 44 percent had up to a 44-bushel loss in yield.”

The researchers collected bags of leaves from the variety testing location and on the U of I campus the inoculant was isolated from those leaves.

Since sweet corn seems to be even more susceptible to tar spot than field corn, Higgins said, the researchers planted sweet corn in September.

“We got about a 1.5-inch rain and the sweet corn emerged and got to about V3,” he said. “And even late in the season, the tar spot infection can take place, so we know the disease cycles.”

The U of I researchers are working to build the data base for tar spot.

“As we get into 2019, if you’re working with a variety plot that has a fungicide component and tar spot starts to show up, let us know,” Higgins said. “Because we’d like to rate it.”

During the past growing season, Higgins looked at a number of fields that had waterhemp weeds that were unaffected by herbicides.

“The weeds have developed resistance to one or more of the chemistries,” he said. “Soil applied herbicides with this weed is a really important component of your weed management system.”

For insect management, Higgins said, yellow sticky traps were placed in three locations last year in fields located near Joliet, Morris and Oglesby. Twelve traps were placed in the fields and four on the edge of the fields for four weeks to determine western corn rootworm beetle numbers.

“We caught two insects,” Higgins said.

“The No. 1 corn insect we have in Illinois is the western corn rootworm but they’re pretty scarce,” he said. “On average there were .11 western corn rootworm beetles per plant across the state.”

In Kane County, the speaker said, he found surprisingly high numbers at one site.

“Be aware if you have farmland in that area,” he said.

However, Higgins said, he saw some interesting insects during the time the traps were in the field.

“I saw bean leaf beetles for the first time in about two years,” he said. “I also saw quite a few stink bugs including the brown marmorated stink bug.”

All stink bugs feed with a straw-like structure that pierces the leaflet and sucks the juices out.

“A brown marmorated stink bug compared to other stink bugs feeds much more aggressively and takes more material out,” Higgins said.

“Even though there were few fields that I visited that reached the economic threshold, the Japanese beetle made a comeback in 2018,” he said. “If you have a late-planted cornfield that is something you want to pay attention to in upcoming years.”

Late in August, Higgins said he found the lady beetle larvae.

“They were feeding on something and we found substantial numbers of soybean aphids,” he said.

“As we go into 2019, to maintain high yields, I encourage you to stay on top of any issue that might limit yields,” he said. “That includes resistant weeds, potential of herbicide damage, downed corn or insect issues — stay diligent.”

Martha Blum can be reached at 815-223-2558, ext. 117, or marthablum@agrinews-pubs.com. Follow her on Twitter at: @AgNews_Blum.

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