Race to the finish to reduce yield impact from tar spot

SYCAMORE, Ill. — Tar spot will be a reoccurring problem in the Upper Midwest.

“The fungus is established here and it can overwinter here, so it doesn’t have to be blown in like southern rust,” said Damon Smith, associate professor and field crops pathologist at the University of Wisconsin.

Tar spot is a polycyclical disease.

“It has many cycles during the season,” said Smith during a presentation at the Illinois Crop Management Conference presented by University of Illinois Extension.

“This is where your challenge is for managing tar spot — not only do we need to worry about it getting started, then we have to worry about secondary cycles,” he said.

Corn plants with this disease will have little black tar spots form on the surface of the leaf and they are semi-imbedded in the leaf.

“The spores are inside the black spots and we think the shape of the spots is related to the hybrid reaction and the environment,” Smith said.

Tar spot first appeared in Illinois and Indiana in 2015, Smith said, however it has been a problem in Mexico and Central America for a long time.

“It was first reported there in 1904 and they manage it with hybrid resistance,” he said.

Smith has grown some of these hybrids in his test plots in Wisconsin.

“There’s some material that’s really quite resistant, but they didn’t make ears because it’s tropical germ plasm,” he said. “Breeders are trying to introduce the resistance, but it’s going to take time.”

When tar spot infects a corn plant, there is a lag phase when the disease is increasing slowly.

“Our strategy with in-season management is to stay in the lag phase as long as we can by trying to reduce the secondary cycles,” Smith said. “When there is a rapid increase of the visible disease, it is too late to apply fungicides.”

Last year in Wisconsin, Smith said, tar spot started in early July.

“By mid-August, it was too late to get ahead of it, so a lot of fungicides got applied and they were a waste,” he said.

Tar spot overwinters on residue and during the growing season wind and rain splash blows it up to the lower part of the canopy.

“There’s the lag phase, then the secondary cycle starts with a rapid increase and the fungus starts climbing up the canopy,” Smith said. “The longer we can delay it climbing up the canopy, the more likely the plant will finish out and yield, so it’s a race to the finish to delay the inevitable with our management strategies.”

Eventually tar spot climbs all the way to the top of the plant and the plant will shut down and prematurely die.

“The fungus can dry the plant out really quickly and cause stalk scavenging,” Smith said. “The plant is still trying to finish the ear, so it has to grab carbohydrates from somewhere and it pulls it from the stalk.”

Weather drives tar spot infections because tar spot loves moisture.

“Without moisture tar spot doesn’t move, but with moisture it moves very well,” Smith said.

Temperature is also important.

“I think we all learned a lesson in 2021 that tar spot doesn’t care if it gets hot for a few hours during the day and that 100 degrees doesn’t seem to be lethal for the fungus,” Smith said. “That will slow it down, and then when it cools off and we have wetness at night, the fungus does its work.”

The Midwest is a favorable environment for tar spot.

“We have the inoculum here and a favorable climate, so we will have epidemics in 2022 and the severity will be dependent on the environment and the hybrids you choose to plant,” Smith said.

Farmers may want to change the hybrids they plant.

“Data shows a yield reduction of 4.8 bushels for every 10% increase in tar spot for the short-day hybrids and for the long-day hybrids there is an 8.2 bushel reduction in yield,” Smith said. “But the longer relative maturity hybrids have a higher yield potential.”

Tar spot loves green material, Smith said.

“It does not like brown stuff, so the longer the leaf is green, the more window of opportunity the fungus has to infect,” Smith said.

“If we plant earlier and a little shorter relative maturity, we might be able to race the fungus to the finish and beat it,” he said.

“You might want to choose a partially resistant hybrid and give up a little yield potential because you won’t have to put on the second fungicide application,” he said. “Maybe the higher yielder is not the best choice, so spend some time looking at the hybrid ratings.”

Smith evaluated fungicides for tar spot in eight environments, including Illinois, during 2019 and 2020.

“Every fungicide gave us some reduction, so you have a wide band of choices,” he said. “Two or three modes of action gave a little better response, so it comes down to cost and availability of product.”

Timing of fungicide application for tar spot is also important.

“In 2019, we did research at six sites and the optimal time was between VT and R2,” Smith said. “The window can shift a little bit from one year to the next, and in 2020, trials showed the optimal time was from VT to R3.”

Smith stressed the importance to avoid spraying fungicides for tar spot too early.

“You can have the environment, but not the pathogen,” he said. “And you may not need a fungicide application depending on the hybrid.”