Tar Spot Ratings Aid Hybrid Selection – DTN – AgFax

Close up view of tar spot on corn leaf. Photo: Martin Chilvers, Michigan State University

Tar spot is a major concern for many corn growers preparing to plant in the spring, and seed companies are increasingly providing information on which hybrids can better tolerate the relatively new yield-damaging pathogen.

All commercially available corn hybrids are susceptible to tar spot, but some are more tolerant than others. Damon Smithassociate professor and plant pathologist at the University of Wisconsin, said most seed companies have or are working on tar spot resistance ratings to help farmers choose hybrids to mitigate potential yield losses from disease.

“The key is to balance yield and resistance, which can be a challenge for some companies,” Smith said. “Farmers need to do their homework to see how good the hybrids are at resisting tar spots.”

AgriGold, an Indiana-based seed company, has provided tar spot tolerance ratings — a scale of one to five, with one being the least tolerant and five the most tolerant — for most of its hybrids for three years. . The ratings are determined by company agronomists who study the hybrids, which will be sold the following year, in research plots in the Corn Belt where the disease is prevalent. Based on the collective results, the hybrids are scored accordingly.

John Brien, Eastern Agronomy Manager for AgriGold, said tar spots dominated discussions at the AgriGold booth at Commodity Classic in New Orleans earlier this month, and at recent grower meetings. . Hybrid disease tolerance has been and will continue to be a hot topic, he added.

“It can be such a devastating pathogen,” Brien told DTN. “In 10 to 14 days, a crop can be destroyed by tar spot. It’s so aggressive.

“We have an extremely high level of comfort with (rating) new products coming out in tarmac environments,” Brien continued. “It gives growers confidence that the produce they are planting is either very good at (tolerating) tar spots or they need to have a solid management plan in place.”

Farmers can find AgriGold hybrids and Tar Ratings by going to its Hybrid Explorer webpage here.

Burrus Seed, based in Jacksonville, Illinois, has released the Tar Spot Ratings for 2022. Find Them here. Hybrids are rated on a scale of one to nine, with one being the least tolerant and nine being the most tolerant of tar spot.

“We thought it was prudent to provide growers with hybrid notes,” said Dana Harder, field agronomist at Burrus Seed. He added that Burrus and other companies are working on breeding hybrids with greater tar spot tolerance.

tar spot in 2022?

Tar spot will likely threaten corn this growing season, just as it will in 2021, according to Smith. He recently delivered the prediction at the Illinois Fertilizer & Chemical Association’s 2022 Winter Convention in Peoria, Illinois.

Where and how severely the yield-damaging disease will affect corn will largely depend on weather and crop management decisions, Smith said.

“I get a lot of questions from farmers asking me, ‘Damon, are we going to have an outbreak (of tar spots) in 2022?’ You bet,” he told farmers and chemical suppliers during a presentation on the disease at the convention. “The severity is going to be related to the (corn) hybrids chosen and the environment.”

Harder and Brien also expect tar spot outbreaks to occur this year. Harder expects the leaf disease to continue to spread in corn-growing states.

“The disease is here to stay,” he said.

That doesn’t mean tar spot is an out-of-control monster guaranteed to reduce corn yields and income. “It depends on how farmers manage it each year and be proactive in the future,” Harder continued.

There are three ways to successfully minimize damage from tar spots if they appear in fields, Brien said. Here are its management keys:

  1. Knowledge. Farmers need to learn about the disease, how it spreads, what it looks like and where it comes from.
  2. Hybrid selection. There are no tar spot resistant hybrids available, but some are more tolerant of the pathogen than others.
  3. Fungicides. The choice and application of fungicides will become more common to control tar spot.

“It’s a new disease that everyone will have to deal with in the near future,” Brien said.


Tar spot first appeared in fields in Illinois and Indiana in 2015, but quickly spread through much of the Corn Belt and a few states on the East Coast and the Middle East. south. It even reached Canada.

According to the Crop Protection Network (CPN), tar spot occurs commonly in the Caribbean, Mexico, and Central and South America. Smith does not know how he arrived in the United States.

Tar spot is caused by the fungus Phyllacora maydis. The fungus produces small, raised black structures (1/16-3/4 inch), round to irregular round and/or diamond-shaped, called stroma. These structures form on the upper and lower surfaces of corn leaves. In severe cases, stroma can also be seen on leaf sheaths, envelopes and acorns.

Look for tar spots that develop during cool temperatures (60 to 70 degrees Fahrenheit), high relative humidity (over 75%), frequent cloudy days, and more than seven hours of dew at night.

“These conditions lead to outbreaks,” Smith said. “But the areas that struggle with not enough water are not high tar environments.”

The pathogen reduces yields by limiting the photosynthetic capacity of leaves and causes rapid and premature leaf deterioration and death. According to the CPN, yield losses can be minimal or zero or 50 or more bushels per acre.

The inoculum overwinters on corn residue. It can survive extreme cold and heat. Rain and high humidity cause the stroma to release spores which are dispersed by splashing rain or wind. Spores can be dispersed in the field and locally. In other words, it is a robust and highly transmissible disease.

“It continues to radiate outward every year and expand its territory,” Smith said. “In Wisconsin, I would say 96% to 97% of our corn acres are infested.”

Larry Bus, a farmer from Logan, Iowa, feels lucky the disease has yet to show up in his fields in west-central Iowa and east-central Nebraska, but he predicts it’s just a a matter of time. Knowing that tar spot affected corn in nearby fields, Buss formulated an action plan this year to mitigate yield loss.

“My plan of attack is to talk to my seed seller about hybrids that not only give good yields but are disease resistant. I will also spot and treat with timely fungicides when needed,” he said.


According to the CPN, several fungicides with 2ee labels can be used to manage tar spots.

View CPN’s Fungicide Efficacy Chart here.

Smith said recent research at the University of Wisconsin and other land-grant institutions shows that spraying fungicides with two or three modes of action works best against tar spots, although one mode of action still offers some effectiveness compared to no spraying at all.

Timing of application is critical, Smith continued. Research indicates that fungicides are most effective during the VT to R3 growth stage. Since the pathogen is polycyclic – it continues to produce spores and spread to new plants as long as environmental conditions are favorable – multiple applications of fungicides may be required.

Fungicides primarily delay the fungus from damaging corn plants to allow cobs to fill out and kernels to add weight, Smith said.

“Some products keep the disease in the latency phase for longer, and that’s where we (preserve) the yield,” he added.


1. Manage irrigation. Reducing the frequency and duration of leaf wetness can reduce the disease, according to the CPN. Excessive irrigation or light and frequent irrigation can increase disease pressure. However, the CPN advised that there is little research on the impact of irrigation on tar spot, and farmers who rely on irrigation should consult an agronomy specialist to determine how irrigation may influence the development of the disease.

2. Rotate crops. According to the CPN, crop rotation seems to play only a minor role in reducing the risk of tar spots. However, this practice will allow the residues to break down and reduce the primary inoculum.

3. Manage residues. Tillage can help, but it seems to play only a minor role in reducing the risk of tar spots, Smith said. Tilling fields bury infected residue and increase the rate of decomposition, which may help reduce the amount of tar spot inoculum overwintering in a field, but will not reduce the risk of infection from locally dispersed inoculum .

“I don’t think we should just moldboard plow,” Smith said.

4. Look for the tar spot. Keeping a close eye on corn can help farmers apply fungicides when needed and be prepared to harvest badly diseased fields early if stalk integrity is questionable to avoid lodging. An online tar spot prediction tool called Tarspotter can provide risk analysis of the disease in certain areas. Learn more about the free Tarspotter app and how to download it on smartphones here.

Find the overview of the CPN tar sport here.

Read a DTN story on the 2021 Tar Epidemic and five takeaways from the season here.

Matthew Wilde can be contacted at [email protected]

Follow him on Twitter @progressivwilde

Martin E. Berry