Tar spot followed heavy rain patterns

GENESEO, Ill. — Reports of the heaviest Midwest tar spot infestations the past five years paralleled heavy rains and humidity.

“In 2018, when tar spot was first recognized in the U.S., there was heavy rain over a 15-day period in August in northern Illinois and southern Wisconsin and we had the heaviest yield-robbing infestation there,” said Ryan Gentle, Wyffels agronomy manager.

There was normal rainfall in August 2019 resulting in minimal yield impact reports from tar spot.

“In 2020, there was a narrow band of 3- to 4-inch rains Aug. 9-15 in northern Illinois and southern Wisconsin. That’s where we saw the biggest yield reduction. There was some tar spot outside of that area, but it wasn’t as severe,” Gentle said.

“2021 is probably when central Illinois, more of Iowa, southern and central Indiana first became aware of tar spot. We got some pretty heavy rains July 11-17, with 6-, 7-, 8-inch rains. Then we had a big rain event in northern Illinois, southern Wisconsin, central Illinois again and parts of Iowa Aug. 8-14 with 6 to 7 inches of rain.

“We saw some huge yield penalties in 2021 where fungicide wasn’t used. We saw 20-, 30-, 40-bushel yield loss in those areas.

“The real wild card in the disease triangle is the environment.”

—  Ryan Gentle, agronomy manager, Wyffels Hybrids

“Looking at the precipitation from Aug. 7-13, 2022, there was a lot of rain in northern and northwest corner of Illinois, northeast Iowa and southern Wisconsin and the temperatures for the most part were 50 and 60 degrees at night where tar spot likes to thrive.

“You had this heavy amount of rain and it was wet for several days and get tar spot started in that type of environment. We saw some pretty heavy yield loss in fields that didn’t get treated.

“If you were outside of that area it didn’t mean you were completely safe. We definitely noticed tar spot pretty much in our whole footprint again. It came on quite a bit later, maybe in the middle of September in 2022, and didn’t cause quite as severe yield penalties if you were outside area that got heavy rain in August.”

“Obviously, where we see these big rainfall events is where we’ve really seen the yield-limiting. It’s really driven by the fact if you think about that micro-climate within the cornfield when you really have that high humidity which you get when you have these drenching rains and you have a lot of moisture sitting in the field. It allows heavy dew to sit every morning and you get that prolonged wetness,” said Mitch Heisler, Wyffels product and agronomy manager.

“Even if you were in an area that had good rainfall in a year you maybe didn’t see the yield-limiting tar spot show up and that’s just because you might have had good rainfall, but not that prolonged heavy dews and high humidity within the cornfield. That’s really the biggest correlation we’ve seen over the course of the last five years here.”

Disease Triangle

A host, which is the corn, and the pathogen and the right environment make up the disease triangle for tar spot and other diseases.

“We know we had the pathogen. In 2021 it was pretty widespread throughout our footprint. It over-wintered very well in residue in the Midwest. So, the pathogen is in the fields. We have corn, the host,” Gentle said.

“The real wild card in the disease triangle is the environment. Are we going to have the right environment? Are we going to have a little bit cooler and wetter environment that will connect all three dots of the disease triangle and make tar spot more severe?

“Another thing we’ve noticed with tar spot is if the plant gets any other stress like gray leaf spot, crown rot or nitrogen deficiency, it really helps that tar spot get established and take over that leaf on the plant.

“Irrigation is highly susceptible. I’ve talked to some industry experts and maybe with your irrigation you can try and put on more less frequently to help let the canopy dry out in between each irrigation pass.”

Spread

Spores can come from residue or be windblown.

“So, even if you do a lot of tillage and bury your residue, if your neighbors no-till, those spores can blow quite a ways with the right storm and be deposited in your field,” Gentle said.

“The disease completes its life cycle in 14 days. So, once it gets established, every two weeks if the environment is right, it’s just doubling and doubling and can explode very quickly.”

Yield Loss

Infection can occur at any growth stage under favorable conditions for disease development, but research suggests infections most commonly start around V8 and become more visible as corn plants enter the reproductive stages.

Yield loss is dependent on the time of the infection and kernel stage development.

“For example, in Illinois this past year it didn’t come in until middle to late September in a lot of cases. We were at the quarter milk-line stage. We think a little past dent is the critical stage where you don’t have to worry about it a whole lot. If tar spot doesn’t appear until quarter milk-line the damage is minimal,” Gentle said.

Identification

Tar spot appears as black raised bumps resembling tar or black paint and can be present on both sides of leaves, as well as on husk and tassel tissues in severe cases. Tar spot cannot easily be scraped off of leaf surfaces using your finger or fingernail.

Spots can appear as a raised black bump on otherwise green healthy plant tissue, or it can be surrounded by a brown to tan halo lesion resulting in a “fish-eye” appearance.

Lesions can coalesce during later development and areas between spots will show a water-soaked appearance and subsequently dry out.

Scouting

“Start scouting for tar spot lesions at V8-V10. If you find lesions, apply a fungicide as soon as possible,” Gentle said.

“At minimum, we recommend a fungicide application by brown silk. If the disease is present at anytime, schedule an application as soon as possible. If there is no disease present, continue to scout and make an application by brown silk.

“Continue to scout after the first application. A second application may be necessary up to dent stage if the disease continues to progress.”

Scouting for tar spot is different than what is done for other diseases.

“A lot of times with other disease you’re going to go in and look at the ear leaf. Tar spot is going to start at the bottom of the canopy. You want to look down towards the bottom of the canopy when you’re first scouting for it to see if you any lesions have developed,” Gentle said.

“We know it’s probably not realistic for a lot of our customers to scout all of their fields every week throughout the summer,” Heisler added.

“One idea that some of our seed reps are doing or heard that others are trying is just pick one field. Maybe you have a corn-on-corn field or one in a low-lying area that might be at a greater risk.

“Pick one field and scout it every week and let it be your kind of canary in a coal mine, so to speak, to monitor it.”

Management

No genetics is 100% resistant yet in the United States. Breeders are working in that direction, but are a ways out.

Timely fungicide applications is the best management practice for tar spot. Start scouting for tar spot lesions at V8-V10. If lesions are found, apply a fungicide as soon as possible.

At minimum, Wyffels recommends a VT-R2 fungicide application regardless of lesion presence. Dual and triple mode of action fungicides worked the best in trials.

Due to the speed of the disease’s development and risk of significant yield loss, a fungicide is worth the investment.

After VT-R2 application, continue to scout. A second application may be necessary up to dent stage, or R5, if the disease continues to progress.

The Crop Protection Network, a multistate partnership of university and Extension specialists, has published ratings for tar spot efficacy on many common fungicide products. The information is published on its website at www.cropprotectionnetwork.org.

Keep Plants Healthy

“If we’ve learned anything over the last two years, the biggest correlation or drivers of when tar spot does become yield-limiting is when there are other stresses involved. Whether that is fertility stress, compaction or other diseases,” Heisler said.

“There were even cases in 2022 where you had two fields across the road from each other with the same hybrid and there was a 30-bushel difference. Really, the only difference was the tillage and the fertility program. Both had fungicides, the same hybrid and same planting time.

“It just proves that sometimes our best defense against a disease like this is just keeping that plant as healthy and happy as possible.”