Emerging Disease: Tar Spot

Thursday, July 25, 2024

‘This is a Good Year to Be Out Scouting & Know What’s Going on in Your Region.’

By Emily Croft

Tar spot is an emerging foliar leaf disease in corn which first appeared in the U.S. in 2015 and was later identified in Ontario in 2020.

As the disease continues to spread and impact Ontario’s corn crop, it’s important that producers are informed about how to identify tar spot and which resources are available to monitor it in their area and beyond.

Tar spot is caused by a fungus called Phyllachora maydis, which produces black, tar-like spots on the leaves of the corn plant. It is somewhat climate dependent, thriving in cool temperatures with high humidity. The disease is transmitted through spores, which are airborne and are now known to also overwinter in residue.

Albert Tenuta, field crop pathologist at OMAFRA, says that the mild 2023/2024 winter leads to questions about how that will affect the spread of tar spot this growing season.

“The question we have this year is, since we’ve had a mild winter, what does that do for tar spot? It could be an issue here in areas where it’s overwintering and in the U.S.,” says Tenuta.

“This is a good year to be out scouting and know what’s going on in your region as well as what’s happening in surrounding areas, including south of the border.”

How it got here

Ontario’s corn crop has been affected by tar spot since 2020.

“Tar spot was first reported in a field near Ridgetown in September 2020, and was later confirmed in fields in Essex, Chatham-Kent and Lambton,” says David Hooker, associate professor in the Department of Plant Agriculture at University of Guelph – Ridgetown Campus.

“The regions with the greatest risk are the counties in the southwestern part of the province, although tar spot has been found in fields as far east as Toronto. The disease has been moving eastward for years, so corn producers across all of Ontario and Quebec need to familiarize themselves with the disease and how to manage it.”

Tar spot was first identified in the U.S. in 2015, in Illinois and Indiana.

Bob Thirlwall, agronomic solutions advisor at Bayer Crop Science, says that weather played a large role in the spread of tar spot to Ontario.

“It was spread to Ontario on storm fronts or weather patterns from the western U.S.,” says Thirlwall.

“The spores were transferred by the wind and landed on the Ontario corn crop and spread from there.”

Regions previously affected by tar spot have an increased risk of continuing to be infected year-after-year, depending on environmental conditions.

“The issue with this fungal pathogen is that it has the ability to produce multiple cycles – we would call it polycyclic – which means that it produces multiple generations of spores that can affect the developing corn crop,” says Tenuta.

“It’s not like a one-off disease that produces one spore and then is done.”

Tenuta explains that the spores overwinter on corn residue and the leaves that are left in the field. This means that once spores are present in a corn crop, tar spot can continue to reappear year-after-year.

Spore movement and the ability to overwinter emphasize the importance of knowing the status of tar spot in Ontario and across the border. Grain farmers should regularly monitor their own crops.

What to look for

Producers across Southwestern Ontario should make a habit of checking their corn crops for tar spot. It is valuable to know what to look for and when to look for it.

“Tar spot in corn is identified by small raised black circular spots that resemble specks of tar on the leaf surface,” explains Hooker.

“These spots are fungal structures called stromata, which are embedded in the plant tissue and cannot be removed by rubbing.”

Thirlwall explains that these black spots cause the yield drag from the fungal infection.

“Because tar spot sets in as a dark black spot on the leaf, it reduces the photosynthetic process,” says Thirlwall.

“This greatly affects yield. In 2023, there was the highest tar spot pressure so far. We saw a yield difference of 30 to 50 bushels per acre for sprayed versus unsprayed fields because of tar spot. It can be a huge yield robber.”

Hooker adds, “Tar spot has reduced yields by up to 70 bushels per acre in fields that were infected early around silking, followed by favourable weather conditions through grain-fill. It was reported on the Crop Protection Network that tar spot was the No. 1 corn disease in Ontario in 2023, causing a loss of an estimated 3.9 million bushels. In severe cases, tar spot can lead to a reduction in test weight and grade due to poor kernel fill.”

Tenuta has been involved with monitoring efforts led by the Crop Protection Network in collaboration with U.S. agronomists, OMAFRA, Grain Farmers of Ontario, and the University of Guelph. In recent years he has been finding tar spot as early as the first week of July.

“Those first two to three weeks of July are critically important. That’s when we start seeing tar spot build up, and that gives us an indication of what’s going on in Ontario, as well as in the U.S.,” says Tenuta.

“In areas with a history of tar spot, it takes six weeks for the initial spores to germinate and build up until we start to see it. If you’re in an area without a lot of tar spot, you’ll likely start seeing it appear higher up on the leaves. The spores would be coming in from other fields or areas, and you’ll see that above the ear leaf.”

Tenuta also says that through his surveillance with the Crop Protection Network and their research into tolerant varieties, they have noticed another type of lesion caused by tar spot. The fisheye lesion has a black tar spot with a tan circle of necrosis around it. He says this lesion appears to occur more often in more tolerant hybrids.

The evaluation of tar spot tolerance, along with other efforts from researchers and crop scientists in Ontario and the U.S., have developed a strong set of tools for managing tar spot.

Managing risk

Tar spot risks significant yield drag in corn crops. The factor that has the largest effect on this risk is weather.

“The spread of the disease is dictated, overwhelmingly, by favourable environmental conditions,” says Hooker.

“If weather conditions are favourable for the development of the disease, and if the pathogen or disease is present in the field, then an application of an effective fungicide may be warranted between the VT and R2 stages of corn development. The fungicide application at this time would offer two to three weeks of protection of non-infected leaf area during the critical phase of corn development and delay the onset of disease development through the grain-fill period.”

Thirlwall adds that fungicide application is the primary approach to managing tar spot.

“You can’t really minimize the spread. If the climate is right, it’s going to be there,” says Thirlwall.

“The main way to minimize tar spot is to use a fungicide that is registered for control of tar spot. Delaro Complete from Bayer is registered for control of tar spot in corn.”

Tenuta notes that in addition to Delaro Complete, it’s been found that Veltyma (BASF), which is registered for control of tar spot, and Miravis Neo (Syngenta), which is registered for suppression, have also shown success in managing the fungal disease.

Thirlwall says that the timing for spraying for tar spot lines up with most producers’ current fungicide programs.

“The best timing for Southwestern Ontario to spray is at R1 timing or Silking, which we also use for protection against ear moulds and DON suppression,” says Thirlwall.

Tenuta says that the Crop Protection Network has also reviewed residue management as a strategy for managing tar spot, but found a well-timed fungicide application was significantly better at reducing the risk to the crop.

“What we’ve seen at our trial locations with very little residue – 20 per cent or less – is that we still observed tar spot development,” says Tenuta.

“Residue management can reduce tar spot slightly, but not to the point where it’ll drastically reduce your risk. You don’t need a lot of tar spot in your field to develop it, and the impact of residue management is limited.”

He shares that the Crop Protection Network has been investigating tolerance in corn hybrids to give grain farmers another tool for managing tar spot.

“The good news is that we have the tools to manage tar spot already. We’ve been doing a lot of work with seed companies and breeders to evaluate hybrids,” says Tenuta.

“We’ve been testing around 100 different commercial hybrids in our nursery or tar spot location at Rodney, Ont., and we see variation in terms of tolerance. We see some with greatly reduced tar spot levels and some that are very susceptible. We are working to make sure this information is available in areas with higher risk for growers that need it.”

As producers monitor their own crop through the summer, it is beneficial to know the status of tar spot in surrounding regions.

Thirlwall says that one easy-to-access option for monitoring the disease is the Tar Spotter app.

“Many agronomists in Ontario are scouting for tar spot in their customer’s fields. There is also the Tar Spotter app to follow the spread of the disease and see if there is a risk in your local area,” says Thirlwall.

The Crop Protection Network has also developed maps on the Corn ipm PIPE website to show yearly historical and current tar spot infection in the U.S. and Ontario.

These tools allow growers to stay up to date on the risk to their fields and help them develop a strategy for fungicide application.

Tenuta says collaboration with researchers in the U.S., who managed tar spot for five years prior to its arrival in Ontario, has yielded information to help Ontario farmers without the usual delay associated with research on emerging diseases.

“Because of our collaboration with the U.S., we were ready to go right away. We had trials lined up and were ready to go,” says Tenuta.

“This really helps us get that info and those tools out to Ontario growers as soon as possible. They don’t have to wait two to three years, because we are already a part of the planning process to deal with diseases such as tar spot.”

By staying informed about the status of tar spot and reviewing the available options for risk management, grain farmers can protect their corn crop against yield losses from tar spot. BF