Search is on for a response to tar spot

Crop diseases often carry a disturbing mix of known and unknown factors.

Take tar spot in corn, as an example. It’s known to have arrived in Ontario in 2018 and has continued to spread across the province, from a few fields in five counties in 2020 to a wider distribution in 19 counties by the end of 2021.

Yield losses in some research plots that year were reported to be greater than 60 per cent.

Why it matters: Although tar spot is more difficult than other fungal pathogens to study, researchers are beginning to learn more about how it behaves.

The unknown factor is how much farther into Ontario it will spread. The disease is relatively new and resistance ratings have yet to be determined. Two fungicides, Veltyma and Delaro Complete (3.83 SC), were rated “very good” from limited sites and locations from 2018 to 2021 by the Corn Disease Working

Group, yet growers must remain vigilant as the 2023 growing season progresses.

One positive bit of news in the fight against tar spot may come from work by Tiffany Jamann, an associate professor from the University of Illinois. From 2019 to 2021, she and a group of collaborators planted 25 corn lines from the United States Department of Agriculture’s Germplasm Enhancement of Maize project and found that two sources, from germplasm originating in Cuba and Brazil, showed resistance to tar spot.

“The great thing about the materials that we tested is that they’re part exotic and part improved germplasm that is close to what is used in breeding programs today,” says Jamann.

“This means that it will take less time to get resistance into breeding lines than if we just used the exotic sources. It also makes them easier to test since they grow well in the (U.S.) Midwest.”

That fits with the urgency of managing tar spot. Crop advisors, agronomists and researchers have more of a chance to learn about the disease and how it works, including talk of longer rotations, increased frequency of scouting and using fungicides with multiple modes of action.

But imparting resistance into new hybrids is a key component in efforts to deal with most diseases. And even if the two sources aren’t exactly perfect — researchers still saw some level of tar spot infection from the two accessions — they did offer a good start.

“We had some suspicions that resistance might be due to different underlying factors in those two accessions, so I think it’s worthwhile to see if something better could come out through the breeding process,” says Jamann.

“This should be combined with additional screening to locate better sources of resistance. There’s definitely the potential to use gene editing to improve resistance to this disease and as we learn more about how the fungus works, we’ll find more potential targets for genome editing.”

Jamann notes tar spot is much harder to work with than other pathogens because it only grows on living tissue, unlike fusarium graminearum, which causes fusarium head blight. That can be cultured on a petri plate or grown on grain.

“The best we can do so far with tar spot is spread diseased leaf material and try to improve the likelihood of disease infection with irrigation,” says Jamann.

“This makes it much harder to work with. We’re dependent on putting the trials in places where there’s a history of disease but that can only go so far. We still don’t always get disease, even in those locations.”

The disease triangle

Previous studies suggested tar spot’s ideal conditions include relative humidity levels above 75 per cent, wet leaves during the night and temperatures between 16 and 23 C. It can also overwinter. Tar spot spores can survive at least one year and possibly up to two years.

“There’s definitely host susceptibility out there, and we’re learning more every season about how the pathogen overwinters and infects the host,” says Jamann.

“But I would say the environment is very important for this disease. We have also learned that there is a lot of diversity in the pathogen, which also contributes to the pathogen’s ability to adapt to new environments.”