A recent survey of 400 farmers by AgReliant and AgriGold shows tar spot is the No. 1 corn disease of concern heading into the 2023 growing season.

And for good reason.

“Tar spot can be very damaging,” Whitney Monin, national agronomy manager for AgriGold, said at Commodity Classic. “Ninety percent of farmers who experienced tar spot in the past experienced some yield loss or profit loss.”

The pathogen, which requires living tissue to grow and overwinters on residue, was first identified in the U.S. and specifically Illinois and Indiana in 2015. It has since spread across the Midwest and been confirmed in all of Illinois except a handful of southern-most counties.

“It continues to move across the Midwest,” said Darcy Telenko, Purdue University Extension field crop pathologist. “The inoculum is there.”

The difference between a major outbreak of tar spot, such as in 2021 when some farmers saw corn yield losses from 20 to 60 bushels per acre all the way to losing half their crop or more, and mild issues like last year centers on the weather and timing of the disease infestation.

“We have problems with yield if (corn) is at the dough stage (at the time of infection),” Telenko said. “It’s important to be out there scouting.”

Farmers should also keep their eyes on weather conditions, which play a key role in determining the severity of a tar spot outbreak any given season.

If the average daily humidity rises about 75% for an extended period of time, particularly during key corn growth stages in fields with a past history of the disease, the chances of a tar spot outbreak increase significantly, according to Missy Bauer, crop advisor at B&M Crop Consulting in Michigan.

“There’s some association of temperatures (to tar spot outbreaks), but it’s much more about humidity,” Bauer said.

“2021 was a firestorm in Michigan. We had dead corn by Labor Day and documented up to 100-bushel yield loss,” she noted. “We felt like we’ve dealt with corn disease before, but we’d never seen anything like this.”

What should farmers do to protect their fields against the prolific pathogen?

It all starts with hybrid selection.

“Assess your risk and talk to your seed representative about hybrid tolerance,” Telenko said.

While no corn varieties currently are resistant to tar spot, many possess varying degrees of tolerance to the disease.

“Hybrid selection is the No. 1 defense (to control tar spot),” Bauer said. “We can’t do this with fungicide alone.”

Mike Popelka, hybrid product breeders manager for AgReliant Genetics, said screening efforts to identify tar spot resistance in corn varieties has increased rapidly since the confirmation of the pathogen in the Midwest eight years ago.

“There’s definitely a difference in hybrids,” Popelka said. “We’re screening and testing through all stages in plant breeding.”

Another tool to manage tar spot is fungicide. But it must be applied prior to or at the beginning of the outbreak, which makes crop scouting critical to managing the disease.

“Scouting is important,” Bauer said. “Once you see (tar spot in a field) from your truck, it’s too late.”

All fungicides labeled for corn provide various levels of tar spot control. In the case of severe outbreaks, multiple applications of fungicide can pay off if the disease is caught in time, Telenko noted.

“Fungicides offer a window of protection,” Telenko added. “But, if you’ve already got an outbreak (of tar spot), hold on to the fungicides, because you’re not going to slow it down.”

Source - https://www.farmweeknow.com