Is it Too Late to Control Tar Spot Now Spreading in Your Fields? Purdue Expert Provides Answers

An aerial view of Indiana’s corn crop this year shows lush green fields just before harvest, a welcome sight after signs and concerns about drought this summer.

“Luckily, because of the dry conditions we had at the end of June and middle of July, it slowed the diseases down that we worried about,” says Darcy Telenko, assistant professor field crop extension pathologist at Purdue University.

Telenko not only studies various plant diseases and the timing of when those diseases sprout in fields, but also how and when fungicide applications can help control those diseases.

She says while dryness caused concern about yield impacts, it was a welcome change in terms of battling plant diseases for Indiana farmers. Rains and other factors caused conditions ripe for increasing disease pressure in Indiana the past few years, specifically tar spot.

“In 2021, the problem was the disease started about July 3 or July 4,” she says.

Tar Spot Had a Late Start in 2022

Telenko digs into multiple corn diseases, and while the first sign of tar spot was reported around August 15 in Indiana this year, the yield hit that some growers saw in 2021 is something that continued to to haunt their management decisions this year.

“When we first started seeing tar spot several years ago, we gave an estimate about 20 to 60 bushels per acre is the [yield loss] we saw, but I bet there were fields last year that had 50% yield losses,” she adds.

Telenko says in years where the disease pops up earlier, multiple cycles then occur after that during the growing season.

“When it starts early, it's going to shut down those leaves and lead to rapid blighting, and if we're not to that black layer timeframe, then we're going to lose some of that weight in that that ear,” says Telenko.

Why Tar Spot is So Difficult to Control 

Tar spot starts in the leaf, and once it consumes the leaf material, the plant shuts down all the leaves that are working to feed the ear and fill the grain.

“The problem is with this disease is compared to the others is it changes rapidly, we go from having nice green corn, you may have 10% or 5% infection and you don't know what's there to two weeks later, the corn is brown,” says Telenko.

But even then, she says fighting the disease with fungicide can only happen if the timing of that application is right. 

“If you've had a consistent problem with it, maybe you need to be ready to put a fungicide out,” she says. “Our work has shown we can go with that standard timing that we go with gray leaf spot at that VT stage, that early tassel to silking. That timing can provide some good yield protection.”

When to Apply Fungicide 

She says automatically apply fungicide at the same time every year may not be necessary if the disease pressure hasn’t popped up yet, or started appearing earlier than normal. That’s why she suggests farmers be proactive and possibly shift their fungicide applications earlier or later, depending on the window of when the disease pressure hits.  

“And last year was one of the years where we could have gone earlier with a late vegetative application and maybe come back in with a second application and protected yield,” says Telenko. “This year, because it came in too late, we may have been able to wait and hold on to our applications and maybe only needed one application to get us to the end of the season.”

Even if you find growing tar spot pressure as late as September, just like growers in Indiana are seeing this year, Telenko says research shows it’s too late to apply fungicide.

“If we're beyond R3, so the early milk stages of the corn, we're not recommending a fungicide application,” she says. “The timing applications that i've looked at when we went out at our four and our five really didn't slow the disease down really didn't give us that yield protection that we were looking for.”

How Widespread Is Tar Spot? 

This map shows tar spot has mainly been concentrated in the upper Midwest, with pockets even on the east coast and southern Georgia, proving tar spot isn’t just a problem Indiana farmers are facing.  That’s why Telenko is also researching whether aerial versus ground fungicide applications show any difference for battling disease pressure like tar spot.

“In some of the work that I and others have done, it doesn't matter as long as you get in the product where it needs to be within that canopy. This disease is going to show you, though, and find those areas that were skipped, or we didn't get enough coverage down. This is the one disease it's going to test all the application equipment,” says Telenko.

From evaluating fungicide efficacy to a NIFA care grant looking at irrigation timing and the impact it could have on the amount of disease pressure farmers face,  Telenko’s research is already reaching beyond these Indiana fields as growers already work toward plans to play offense against potential diseases next year.

"We've shown that it least over winters for one year in the plant debris," says Telenko. "We don't know if it's more than one year. So I've been in fields here in Indiana before where we really had a widespread epidemic, where the the field had tar spot in the previous corn with soybean corn rotation. So, it is survived at least a couple years before the disease ramped up and caused yield losses," she says. 

Telenko advises producers to know the risk on their own farm, and that may vary by field. She says that will help growers prepare for the next growing season.