Farmers ought to scout for tar spot in corn | Agriculture

“It’s definitely a disease we want to have our eyes on and know what’s going on with it,” said Bish, University of Missouri assistant professor and state Extension crop pathologist.

“You’re looking for tiny black tar-like spots, or stroma, on corn leaves. Initially there will only be a couple of them, but they go through the leaf surface. They’re different than insect frass, the thing most commonly confused with tar spot this early in the season.”

Tracking disease incidence and spread help determine when to treat fields.

“Our goal is really not to jump the gun on fungicide applications or apply too early at very low disease incidence,” Bish said. 

Treating too early with fungicide could require a second application, and with crop condition already a concern due to the dry weather, “people don’t want to pay to spray fungicide on corn this year,” Bish said.

Already observed in Illinois and Indiana, tar spot was found in Missouri in Lewis County in 2019. The disease now has “enough inoculum present to cause yield losses” across the state, Bish said.

When tar spot lesions cover around 30% of the ear leaf, the leaf can prematurely die, but yield reductions can occur at much less severity.

“We are still learning about this disease,” Bish said. “We know once you have it, the pathogen can survive the winters here. Now we know it is capable of infecting our corn crops in Missouri in June.”

Bish said June’s weather likely helped spur the disease, just as July’s weather so far may help control it.

“We’re not sure how the fungal pathogen will react to warmer temperatures,” she said. “It’s a disease that typically favors cooler air temps. Our June evenings have been very cool, probably four degrees cooler than normal and dew point temperatures also are cooler.”

Fungicides with two or three modes of action have been most effective against the disease in most small plot research studies. Producers can sign up for text alerts tied to the disease online at ipm.missoui.edu.

Bish said farmers also need to be aware of southern rust, a disease that doesn’t get as much attention at tar spot but can cause just as many problems.

Southern rust “cannot overwinter in Missouri, so the spores most up from the south each year,” Bish said. “Southern rust can be confused with common rust. Usually the rust-like symptoms appear on upper leaf surfaces, with common rust on both leaf surfaces.”

Submit suspected samples of tar spot or southern rust to the MU Plant Diagnostic Clinic for confirmation and to help track progression of the disease. Information about submitting samples is available online at plantclinic.missouri.edu