What is 'tar spot' disease and why is it spreading in Kansas corn fields?

TOPEKA (KSNT) - Plant experts say a disease targeting a major cash crop in Kansas is making a comeback in several counties this year, negatively impacting the state's farmers.

Rodrigo Onofre, a plant pathologist with Kansas State University, said in a recent publication on Monday, June 24 from the university the fungal disease commonly known as tar spot, phyllachora maydis, is being detected in corn fields located in four counties this year. Tar spot appears as small, black, raised spots on the leaves of infected plants and can lead to yield losses. Recent increased rainfall is believed to be behind the spread of this disease.

A separate report from K-State's Agronomy eUpdate on June 6 stated Kansas is the first state where tar spot is being found in 2024. The disease lead to severe corn crop losses in the counties where it was found last year.

In 2023, tar spot afflicted corn production in 12 counties, all located in northeast Kansas, causing problems for some of the state's farmers. Counties which currently contain the fungal disease in 2024 include the following:

• Doniphan
• Atchison
• Jefferson
• Nemaha

All of the above counties were included in last year's list. However, K-State said the disease is showing up much earlier, around a month's time, than it did in 2023. Onofre is recommending farmers keep an eye out for tar spot and treat it quickly with fungicides to stop it from spreading.

“If you wait until significant disease is in the upper canopy, a fungicide application may be too late (to help the crop),” Onofre said. “The early onset of disease that we’re observing this year raises concerns about yield loss. Generally, early observations of tar spot have corresponded with high yield loss. The recent rains (in that region of Kansas) likely helped to promote development of tar spot.”

Onofre said repeat applications of fungicide may be necessary to curb the growth of tar spot. Scouting and disease monitoring activities are also important for controlling its spread.

“Research has shown that the best return on investment from a fungicide application on corn occurs when fungal diseases are active in the corn canopy,” Onofre said. “A well-timed, informed fungicide application is important to reduce the disease’s severity when needed, and we recommend holding off until the disease is active in your field and corn is at V10-VT growth stages.”

Tar spot is also being reported in nearby states like Nebraska, Iowa and Missouri. An online map showing where tar spot was reported last year and where it is being reported this year can be found here. If you have questions about tar spot and what to do about it, call Onofre at 785-477-0171 or send an email to onofre@ksu.edu.

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