Crop rotation, residue management, hybrid selection and appropriate use of fungicides are all part of the arsenal growers in Michigan, Ohio and Pennsylvania employed in this year’s battle with tar spot.

The fungal disease was widespread in Michigan, appearing in all major corn growing counties — in the lower half of Lower Michigan. Ohio had only a few reports in the northwest, while Pennsylvania’s Lancaster County is again the lone infected county in the Keystone State and the only area in northeast and mid-Atlantic to report it.

The disease came in early July in Michigan and Ohio, but it wasn’t detected in Pennsylvania until the last week of August.

Tar spot is caused by the fungus Phyllachora maydis and can be identified by the raised, black spots that appear on corn leaves and husks. Surrounding some of the black spots may also be a tan halo, which is called a fish-eye lesion. Each one of those black tar spots is the reproductive structure of the tar spot fungus, which can produce thousands of spores.

Research has demonstrated that the tar spot fungus can overwinter in those black spots, releasing spores the next year, says Marty Chilvers, field crop pathologist with Michigan State University. Cool, humid conditions with prolonged leaf wetness can lead to higher tar spot pressure in fields. Windy and rainy conditions may also spread spores to new fields, or new areas within a field. Chilvers advises that irrigators should be careful they do not promote tar spot with light frequent irrigation, which may increase leaf wetness events.

The tar spot fungus appears to overwinter in infested crop debris, although the exact means of how the fungus overwinters, and the exact way it infects, are not known.

Ohio State University Extension reports that fungicide applications made at silking (R1) or tasseling (VT) are the most effective in terms of foliar disease control and yield response in Ohio. 

Joe Timmer, sales manager in agronomy at Farmer’s Cooperative in Zeeland, Mich., says even though it came early, weather held tar spot back a bit.

“We had a less humid and sunny July, which helped,” he says. “The pressure stayed light for July and early August, but then it got cloudy and humid with foggy mornings, which really ramped up the disease. We started spraying mid-August, which seems to have halted the disease progress.”

Timmer says he advised growers to watch tar spot closely, and if it stays suppressed, to wait as long as possible before apply fungicides. “But once it starts to ramp up, go out and nail it quickly,” he says. “It’s hoped the fungicide would provide protection later into the season and get away with one application.”

But before anything, Timmer says to give it a whole-systems approach and first build the strongest, healthiest plant possible, and pick a variety with the best tolerance possible.

In the past, losses have been upward of 40-50 bushels per acre, he says. “That was when it first came around, and we weren't looking at it as hard as we are now,” Timmer says. Last year, the disease came in late and had no effect on yield.

Tar spot, which has been prevalent in Mexico, Central America, South America and the Caribbean, was first detected in the U.S. in 2015 in northern Illinois and Indiana. This year, it is in Illinois, Iowa, Indiana, Wisconsin, Michigan, Ohio, Pennsylvania, and one county each in Missouri and Minnesota.

Tar spot was first discovered in Michigan’s Eaton County on July 1, and then in Allegan and Montcalm counties. On July 7, tar spot was confirmed in an irrigated field in St. Joseph County. Chilvers says this is the earliest confirmed case since the disease was first discovered in Michigan in 2016.

Spreads in waves

Despite finding it early, tar spot spread in waves and at different times.

“It came in late here, around R3,” says Cade Klein, who farms in Marcellus, Mich., in Cass County. “We sprayed July 30 when corn was at or past 50% brown silk with HeadlineAmp, as supply was tight on a lot of products, and HeadlineAmp was recommended. I believe it saved us bushels, but it definitely didn’t prevent us from infection. My initial thought was that the plants were far enough along to escape tar spot, but it appears to be spreading quickly. I think every field in the area has it — fungicide or no fungicide.”

From fields Klein has seen, he says Aproach Prima and Veltyma appear to have the best control. “But, with the high humidity and foggy-dewy mornings we’ve had this month, no field is tar spot free,” Klein adds. “All in all, tar spot can take the wind out of our sails after thinking we were on track for a great crop. But we still have a ways to go to get this crop finished, and the combine will tell the tar spot tale.”

Randy Poll, who farms 3,000 acres, including 1,900 of corn in the Hamilton, Mich. area, (just south of Holland and 10 miles from Lake Michigan), sprayed 90% of his corn in late July.  

“In past years, we’ve noticed that where we sprayed early faired a little better,” he says. “So last year and this year, we’ve applied a preventative spray — it’s been my experience that if you find out you have tar spot, it’s too late. It can rob you of 30 to 40 bushels per acre.”

Poll feeds all his grain, suppling his 45,000-head farrow-to-finish hog operation. “Quality is a huge deal; we needed to control it because it hits faster than you realize,” adds Poll, who sprayed Delaro Complete with three modes of action. “Three years ago, we saw it and didn’t think a lot of it. But, in three weeks, it killed our corn. We lost test weight. The quality of corn went down, and stalks started to deteriorate.”

Pennsylvania infection

A neighboring field to last year’s infection in Lancaster County, Pa. (northwest corner) is where the infection was reported this year, says Jeff Graybill, Penn State agronomy educator.

“We had a dry June and July, which seemed to prevent inoculation from getting a toe hold,” he says, while adding that producers should be scouting for both gray leaf spot and tar spot. “With all this rain of late, it’s fueling disease pressure. We’re urging growers to keep track of it and let us know. For next year, growers should check with their seed dealers to seek out varieties not as susceptible to tar spot infections.”

To date, tar spot has been observed most often during mid- to late grain fill (growth stages R3-R6) and usually on leaves below or near the ear leaf. You can observe stromata in green and senesced tissues. Occasionally, you may also observe necrotic brown tissue surrounding the black structures, which produces a fish-eye appearance.

If you suspect tar spot, The Crop Protection Networks advises sending a sample to your state diagnostic lab to confirm the diagnosis.

Lessen your risk

Several management practices may help reduce tar spot development and severity. CPN offers these tips:

Manage residue. Tilling fields buries infected residue and encourages it to decompose, which may help reduce the amount of overwintering tar spot inoculum.

Rotate to other crops. This will allow residue to decompose and reduce the primary inoculum. It is not yet known how many years it may take to sufficiently reduce inoculum.

Avoid highly susceptible hybrids.

Investigate fungicides. Some fungicides may reduce tar spot, and there are several fungicides with 2ee labels that can be used to manage tar spot. However, there is little data about application timing that will provide an effective and economical response. Efforts are underway to understand the biology and epidemiology of this disease, which may help formulate fungicide application decisions in the future.

“I agree that rotation is important,” Chilvers says. “And certainly corn-on-corn would be at slightly higher risk. But I rank these very low in terms of having impact on disease. The spores move around in a region, and we’ve seen fields that have never had tar spot get smashed with the disease. It has also shown up in fields that have not had corn in them for many years, and in fields with good [long] rotations. The most important components are susceptibility of the hybrid and weather conditions.”

Rust or tar spot?

Usually, lesions from rust diseases are initially bright orange or red, but as the season progresses, lesions may turn dark brown or black and can be mistaken for tar spot, according to CPN. So, how do you distinguish corn rusts from tar spot?

The CPN says use a 10-power hand lens to examine the lesion. Rust lesions erupt through the leaf surface, and you can rub off the spores. Tar spots are raised and black, and do not leave a smudge when you rub them with your finger.

What to do if you find or suspect tar spot

Michigan growers can email pictures to [email protected] or via Twitter @MartinChilvers1. Physical samples can be submitted to MSU Plant & Pest Diagnostics, Center for Integrated Plant Systems (CIPS), 578 Wilson Road, Room 107, East Lansing MI 48824-6469.

Ohio growers are asked to inform your state specialist, field specialist or county Extension educator, but most importantly, send samples to the lab at 1680 Madison Ave., Wooster OH 44691, for confirmation.

Pennsylvania growers seeking a diagnosis should contact their nearest Penn State Extension educator to obtain a positive identification and alert state specialists Alyssa Collins, 717-653-4728 or email at [email protected], or Paul D. Esker, 814-865-0680 or email at [email protected].

Samples can be sent to 111 Ag Analytical Services Lab, University Park, PA 16802.