Agronomists in the U.S. Midwest are already fielding questions about it.

It’s not to say tar spot has become the most significant disease in the crop but it’s certainly garnering increased attention from growers and their advisors this year. American agronomist Ken Ferrie has been getting calls from farmers concerned with the arrival of tar spot, well beyond his own local area of central Illinois into parts of Iowa, Minnesota and Wisconsin. Most of their questions deal with identification and whether they need to spray.

Unfortunately, Ontario growers need to be on their guards for the disease; it’s been confirmed -earlier than normal -in Oxford County as of July 12th, and those north of London are advised to begin scouting.

Tar spot first appeared in Ontario in 2018, making 2024 the seventh year farmers are dealing with the leaf disease. First confirmed in the four southwestern-most counties, it’s spread to the western edge of the Greater Toronto Area, detected in 19 counties by the end of 2022. Yield losses in some research plots exceeded 60 per cent.

Keeps shifting
Part of the challenge with tar spot is its causes and capabilities continue to adjust to changing conditions. Five years ago, the fundamentals of integrated pest management (IPM) were thought to be sufficient in keeping the disease relatively in check. But rotations and incorporating residues with tillage couldn’t counter the pathogen’s ability to overwinter not just one winter, but sometimes two. In the past few years, there’s been sufficient inoculum in the soil or it’s blown in from neighbouring fields or neighbouring states.

Since there are no specific levels of resistance to tar spot in the current line-up of hybrids available to growers, the key recommendation from BASF’s Rob Miller, is to select those hybrids that are less susceptible.

“Proper hybrid selection with a well-timed fungicide can help manage risk, “says Miller, BASF’s regional technical services manager for Eastern Canada. “Looking at corn management as an entire package, not just trying to manage one disease, is very important. Every year is different but we continue to learn a lot for both tar spot to come up with better recommendations.”

Another issue that’s shifted in the past couple of years is an accompanying infection from northern corn leaf blight, similar to how soybean cyst nematode (SCN) is combining with sudden death syndrome (SDS) to reduce soybean yields. In parts of Ontario in 2023, northern corn leaf blight was the prominent disease followed closely by tar spot, which caused a rapid shut-down in the plants within a couple of weeks.

Ontario’s hot and humid summers often provide the perfect conditions for tar spot to flourish. Optimum conditions include temperatures between 16 and 23 C and relative humidity above 75 per cent. In 2023, Ontario had higher levels of tar spot than the U.S. Midwest.

“It’s this constant battle trying to find the best recommendation as the disease can vary year to year based on the location,” says Miller. “We continue to provide more precise recommendations based on the environment, crop stage and risk level. But it’s nothing that comes quickly, especially given the newness of the disease.”

He adds they’re still recommending applying fungicides at tassel but if the disease is confirmed in a field, the grower can still apply up until R3 to R4. Up to that point, they were still seeing economic response to spraying, even in susceptible hybrids. But keep in mind that fungicides always work better when applied as preventatives and are just one “tool in the toolbox” to manage this disease.

Identification is key
The first step in dealing with a problem is identifying it, and determining whether tar spot is in a field can be confounded in several ways. First, the disease only grows on living tissue, manifesting itself in mycelia, spores and stromata -the black raised spots that form on the leaf surface. Research in the U.S. has found that fungicides used for tar spot will kill the mycelia and spores of the disease but not the stromata, which are the long-term structures that survive and produce spores. When scouting fields, test leaves by wiping a gloved finger across the suspected tar spot stromata: if they can be scraped off, it’s likely just insect frass (excrement). If it stays on the leaf, it could be tar spot.

One researcher in the U.S. has identified a source of resistance to tar spot from the same organism that’s helped control tan spot in wheat and sclerotinia in soybeans, however that work is still in its earliest stages. Finding a biological control and imparting a suitable trait could take up to 10 years before a resistant hybrid line is available.