Tar spot, a disease of corn plants caused by the fungus Phyllachora maydis, has been causing problems in Mexico since the 1910s, but it stayed out of northern climes for more than a century. So it was surprising when tar spot was discovered in cornfields in Illinois and Indiana in 2015. The following year, it showed up in Wisconsin and a few other Midwestern states.

“Everybody thought it was a subtropical pathogen at the time,” recalls Damon Smith, professor and extension specialist in the Department of Plant Pathology. “People just sort of said, ‘Well, it’s cosmetic, it’s not going to be a problem.’”

Then the epidemic of 2018 happened. During that growing season, U.S. farmers lost approximately 5 million metric tons of corn to tar spot, an economic setback of around $680 million.

“We were just running all over the place, helping farmers try to make decisions plus gathering some data for research,” Smith says. “There was very little out there in terms of what we could do to manage it. We got caught without any solutions.”

In subsequent years, the disease continued to expand its territory. Smith assembled a multi-state team to study the new pathogen, and they secured funding in 2020 via the National Predictive Modeling Tool Initiative, a program of the USDA Agricultural Research Service, which supports the development of research-based tools to forecast U.S. crop diseases.

The photo shows a corn leaf that has been infected by tar spot. The leaf has turned mostly brown and is covered in black spots. Tar spot on corn leaf blades.

Tar spot on corn leaf blades.

For their first step, they identified the weather conditions that promote tar spot development. Smith and then-graduate student Wade Webster PhD’22, along with collaborators in eight states, began scouting corn fields for the disease. Webster then gathered the team’s data, standardized it into a single format, and worked on the analytical aspects.

“We examined the correlations between tar spot development and over 100 environmental factors,” says Webster, who is now a faculty member at North Dakota State University. “With that, I was able to explore various statistical models to predict the risk of disease development.”

The team published their findings in Scientific Reports in October 2023. Even before that, as they were working on the manuscript, they took the project one step further, spurred by their dedication to the Wisconsin Idea — the long-held principle that the work of the university should benefit the citizens of the state and beyond.

“[We wanted to] help the farmers who are actually doing the work,” Webster says. “It’s all about getting the best tools into their hands, making their lives a bit easier.”

The result was an app called Tarspotter, built by staff in the Nutrient and Pest Management (NPM) program at UW–Madison and released in spring 2023.

“We take really complicated math, and we cook it in the background on the phone, and then we kick out nice displays and things for the user to look at. And it all runs in seconds,” says Smith, who has been working on plant disease prediction models for 20 years and is an affiliate at UW’s Data Science Institute.

The developers sought to make the app as smooth and accessible as possible — an effort that users have noticed and appreciated.

“The app is so well thought out. You click a few things, you put in minimal information, and it works,” says Eric Birschbach MS’91, owner and operator of Ag Site Crop Consulting. “I work in a three-county area with different farming systems, and I can pull up all the fields I’ve got in there, and I can press one button, and it updates every field to tell me what the risk is in each of those fields, considering each microclimate — which is cool.”

Given the app’s success, Smith and his NPM collaborators decided to take yet another step. They cofounded Field Prophet, a startup company to ensure the longevity of Tarspotter and the other crop disease apps they’ve developed.

“Funding agencies can’t always support the long-term maintenance of tools like this,” says Smith. “So we said, ‘OK, let’s do this.’ ”

The Field Prophet app, which is free, has been downloaded more than 7,000 times, and last year it received around 300 hits per day during the growing season. It tells users whether they need to spray fungicide; and, when spraying is needed, the app helps them optimize the treatments to maximize the return on investment in the product. It’s a win-win-win because it’s better for the environment and farmers’ finances, and it helps prevent pesticide resistance in crops.

When it comes to optimal conditions for tar spot growth, what does all that data the research team collected tell the app to watch out for? Counterintuitively, the pathogen doesn’t need maximum heat and moisture to thrive. In fact, it prefers extended periods of mild temperatures — 30 days of around 70 degrees Fahrenheit. It likes a heavy rain followed by a long stretch of relatively dry conditions.

And it seems to always find a way.

“Sometimes it comes early, sometimes it comes late. Most years, though, you can walk into any field at harvest time and see tar spot,” Birschbach says. “So, it’s great to have this tool so we can actually use some integrated pest management to try to make good decisions.”

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