Tar spot takes root as top corn plant disease

Ray Mueller
Tar spot on a leaf of corn located in Arlington, WI on August 7, 2019.

FOND DU LAC – With tar spot apparently having established itself as a permanent late season disease threat to corn in Wisconsin, Extension Service plant pathologist Damon Smith wants to update his data for where it has appeared in the state.

Speaking at the 2019 pest management update meetings, Smith asked growers and crop consultants from the counties bordering Lake Michigan to share samples of corn plant residue for testing if there was any suspicion of the disease in the field. This would augment the documented outbreaks of various severity in multiple counties in the southern half of the state in the past few years, he noted.

During 2019, tar spot was also found in all but the far western counties in Iowa, far southern Minnesota, the northern half of Illinois, much of Indiana, southern Michigan, and one county in Ohio. The disease thrives on wet corn foliage, as was illustrated in the photo of an irrigated Michigan field even though the field was treated with a fungicide as the corn was tasseling.

Damon Smith

Effects on yield

Smith and other researchers have been monitoring those infestations for the effects they have on corn grain yield. They have learned that the effect differs greatly between corn hybrids, that later maturity hybrids suffer greater yield losses, and that the tar spot pathogen can overwinter in the Midwest.

For infected corn in Wisconsin, Michigan, Illinois, and Indiana in 2018, average per acre yield losses were 4.5 bushels for 92 to 106-day maturity hybrids and 8.2 bushels for 107 to 113-day hybrids for every 10% point increase of infected plants.

Based on more than 500 data points through 2018, yield gains of up to four percent were calculated with fungicide treatments if the timing was good, Smith reported. For 2019, a product test, which he termed as a “spray rodeo,” was carried out with 24 fungicide formulations.

While one or two applications could protect yield in some situations, the overall averages on achieving a payback on the costs are not favorable, Smith observed. With two applications, the odds of at least a breakeven on yield gain versus cost are less than 50%, he indicated.

Control possibilities

What's troubling is the discovery, during testing of crop residues in Illinois in the spring of 2019, that the tar spot pathogen can survive during the winter, Smith remarked. He said that plowing infected fields in a solution to the disease.

Another recent and troubling observation is that some plant infections are starting at the top rather than at the bottom, which had been the previous common occurrence, Smith stated. Infections starting at a plant's top suggest that the spores are being spread through the air rather than only from the soil surface, he explained.

Other disease concerns

Despite the rapid onset and spread of tar spot, “do not be caught in a tar spot tunnel vision” regarding corn diseases, Smith warned. With the late growing season for much of the state's corn and with many acres not yet harvested for grain, there's a strong possibility of gibberella ear rot and associated vomatoxins in the grain, he pointed out.

Other fairly common diseases that were identified in Wisconsin during the 2019 growing season were gray leaf spot and northern corn leaf blight, Smith noted. Both anthracnose and gibberella stalk rot also struck in some fields, he added.

More details on the research trials involving disease effects, efficiency of multiple fungicides, and the breakeven numbers on yields related to treatment costs are on the website.

Smith can be contacted by e-mail to or by phone at (608) 286-9706.