PEORIA, Ill. — Tar spot (Phyllachora Madis) fungus on corn is an increasing concern for farmers in northern Illinois, Indiana, Wisconsin, Michigan and other states and regions that experience higher moisture levels. What to do about tar spot is a subject that is still under study by agronomists across the upper Midwest. 

Carefully selected corn hybrids defended with strategically timed applications of fungicides provide the best known protections against tar spot, according to Dr. Damon Smith, associate professor and Extension plant pathologist for the University of Wisconsin-Madison (UWM). Smith presented the latest research conclusions regarding tar spot to farmers and retailers during the Illinois Fertilizer and Chemical Association’s annual convention on January 18. 

“Tar spot has been isolated as one fungus, but as recently as 2018 we weren’t sure. Since then we’ve learned a lot,” said Smith, who holds a B.S. in biological sciences and has also worked as an associate professor and plant pathologist at Oklahoma State University. “For all intents and purposes here in the Midwest, we’re dealing with just one organism. But it can move quickly, and it’s a significant problem that is taking a lot of yields.”

Smith explained that recent research indicates there may be endemic (native and restricted to a certain region) species of Phyllachora. While only one species has been discovered in the upper Midwest, two species of P. Madis are known to exist: a “Mexico” population that is closely related to isolates found in parts of Indiana, Ohio and Florida, and a “Caribbean” strain identified in Illinois, Wisconsin, Michigan and elsewhere. However, the two species could soon converge their territories. 

“Are we going to have an epidemic (of tar spot) in 2022? You bet. The severity is going to be related to the hybrids we choose and the environment we get during the season,” Smith predicted. “We are dealing with a polycyclic disease, which means it has several cycles during the season. That and the fact that it moves so quickly is what makes it so challenging.”

Research conducted in the upper Midwest has shown that tar spot can overwinter in crop fields, making tillage and crop rotation a near-must in areas prone to the fungus. In addition, studies show that areas with heavy irrigation usage can experience more tar spot fungal growth than in areas with more non-irrigated fields.

Smith said tar spot has been found to thrive in regions that have an average monthly temperature of 63 to 72 degrees Fahrenheit, an average RH greater than 75 percent, experience at least 7 hours per night of leaf wetness and 10 to 20 foggy days per month, and have an average monthly rainfall total of at least 5.9 inches.

Smith recommended the free TarSpotter mobile app, available for iPhone and Android phones, to farmers who want to diagnose tar spot probability in their corn fields. Developed by the UMW, TarSpotter uses GPS coordinates to determine if weather has been favorable for the development of tar spot fungus during corn flowering in a specific field. Models in the app use local weather (gathered through the internet) to predict favorable conditions for most corn growing regions to generate site-specific risk predictions.

“It tells you if you need to get out there and do some scouting,” said Smith, who also recommended the FieldProphet app and website, which utilizes tar spot models to create longer-term weather risk prediction scenarios.

University research suggests that untreated tar spot becomes apparent around the R3 corn growth stage, making the V8 to R4 growth stage the best time to manage tar spot. “By the time we get to R5 we can see a pretty high disease level, but a resistant hybrid can give you a nice reduction,” said Smith, who recommended farmers consult with their seed dealer when selecting resistant corn hybrids. 

UWM researchers studied 13 fungicide products from manufacturers including Headline, Veltyma, Lucento, Revytek and others for efficacy against tar spot. The results, published just last week, were summarized for those in attendance by the extension plant pathologist.

“All the fungicides showed a significant reduction in disease across all environments, but yes, some products did a little better than others. What you will notice is that the most effective products had two or three modes of action — not all of them, but most.” 

A summary of the 2021 UWM fungicide study on tar spot efficacy including product ratings, can be found at: https://badgercropdoc.com/wp-content/uploads/sites/5/2021/12/2021_FungicideSummary_final.

Another just-issued study on fungicide efficacy for tar spot and other crop diseases conducted by the Corn Disease Working Group and adapted for Indiana can be viewed at www.extension.purdue.edu/extmedia/BP/BP-160-W.pdf.