As corn planting looms, the dread of disease pressures may be occupying producers’ thoughts.
Some of the pesky problems in these parts are relatively new to the headache list, including Tar Spot, Southern Rust and Corn Stunt.
Those diseases attack corn plants by limiting photosynthesis, but in different ways, robbing them of yield potential. Pictured above is young corn plants by Nemanja Otic, Adobe Stock.
Staying vigilant in the use of available strategies, are paramount, experts say.
Tar Spot
Tar Spot develops as small, black, raised spots, either circular or oval in shape, and may appear on one or both sides of the leaves, sheaths, and husks, Kansas State University plant pathologist Rodrigo Onofre, PhD, told K-State Research and Extension’s Pat Melgares last June.
Tar Spot has now been confirmed in 15 Kansas counties, Onofre said, including Atchison, Brown, Doniphan, Jefferson, Nemaha, Marshall, Washington, Riley, Pottawatomie, Leavenworth, Douglas, Wabaunsee, Coffey, Woodson, and throughout the Corn Belt.
“(The disease) has been in the U.S. for about 10 years now,” Onofre said. “Since 2015, Tar Spot has cost millions of dollars in corn yield loss. Under a severe outbreak you can see losses of more than 60 bushels per acre, also in forage quality, reduction in ear weight, and lodging at the end of the season.”
Losses were worse in 2023, in comparison to the 2024 season, he said.
Experts have developed ways to deal with Tar Spot, such as targeting opportune times to attack the disease with fungicide.
“We’ve reached a point of knowing what different hybrids will do from a susceptibility standpoint, and what fungicide works,” said David Hallauer, K-State Extension agent in the Meadowlark District that covers Jackson, Jefferson and Nemaha counties in northeast Kansas.
Early identification is important, Onofre said, to outline a management plan.
“Hybrid selection is one of your best tools, in combination with fungicide,” he said. “A lot of fungicides have shown great efficacy. Two and three modes of action is better than one, but one is better than nothing.”
When Tar Spot shows, Onofre said it’s a good idea to submit samples for examination in diagnostic labs.
The best time to apply fungicide is while the fungal diseases are active in the corn canopy, he said, and while the corn is between V10 (with 10 visible leaves) and VT (when the last branch of tassel is visible, the plant is at its full height and about to release pollen).
A second application might be needed between the R1 (silking) and R3 (milk)—reproductive stages in corn development.
Scouting and disease monitoring are crucial for best disease control, Onofre said.
There has been no confirmed presence of Tar Spot on Justin Knopf’s farm or southeastern Saline County, or in other areas of the county, but the Pioneer seed dealer said, “Hybrids vary on levels of resistance, so hybrid selection is an important consideration where the disease is present in northeast Kansas and east of the Flint Hills.”
Southern Rust is new to Middle America over the past five to 10 years, he said. Nebraska Extension Publications reported that Southern Rust tends to cause 45% more yield loss than common rust.
“The fungal disease over-winters in the South and blows up here when we have strong southern winds in the middle to late part of summer,” Knopf said.
Wind currents or water carry the infected spores, according to earthdevelopmentinc.com.
Southern Rust
Southern Rust damages corn by taking over the green leaf tissue, reducing the plant’s canopy and inhibiting photosynthesis, Knopf said, bringing on poor late season plant health, affecting yield and harvest standability.
“If it comes early enough and is not treated, it can lead to lodging,” he said. “We should be paying attention to local agronomists. (Seed dealers) help our customers scout fields regularly. K-State Research and Extension does a nice job of monitoring the presence of diseases.”
If Southern Rust is present or has been spotted south of your fields, Knopf said, scouting and fungicide applications are effective at stopping the spread.
As a dryland farmer, he said, Southern Rust has been spotted in some fields, “but it hasn’t been here early enough for us to treat.”
Balancing the probability of yield damage is necessary to determine whether to take action.
“The last couple of years, we’ve been more hurt by drought or heat than we have by diseases,” Knopf said. “Several customers with irrigated corn have certainly treated for the disease over the past several years, and it has been the right decision agronomically.”
Corn Stunt
The invasion of Corn Stunt disease that damages yield through the reddening of leaves and the entire plant, and yellow stripes, as well as stunting the growth of corn plants, has been fairly widespread.
It was reported in 26 Kansas counties last season and in nine other new states, including Oklahoma, Arkansas, Missouri, Nebraska, and South Dakota, said K-State Extension’s Onofre.
“It’s been around since the 1940s in Florida, Texas and California,” he said, “but has never made it to the Midwest and northern states.”
Corn Stunt disease is transmitted by the corn leaf-hopper.
“This is a new bug, a different critter,” said Jay Wisbey, agricultural Extension agent for Saline and Ottawa counties in north-central Kansas.
“(The disease) is something you can’t control with a fungicide,” he said. “Now we’re just trying to figure it out.”
Onofre added that both the disease and the leaf-hopper is puzzling. The challenge of insecticide, he said, is learning what and how many times to spray.
“Kansas Corn has funded our research, and we will be monitoring the leaf-hopper and Corn Stunt disease in partnership with the Kansas Association of Independent Crop consultants, K-State Extension crop agents and Corteva Seed Company,” he said.
Corn Stunt is a new foe, said Dusty Fanning, independent crop consultant from Sublette, in southwest Kansas.
“We’re all learning this together,” he said. “Last year would’ve been our first encounter. It showed up late in the season and had us on our toes. A lot of crop consultants are trying to get a handle on what we’re seeing.”
In fields where 1% to 3% of the plants were affected, Fanning guessed Corn stunt cost “a bushel or two of yield loss (per acre) at most,” last year.
“Those guys with 20% to 40% of the plants infected had more noticeable yield loss,” he said. “Reading articles out of South America, if they get early infections, it could be devastating.”
Kansas experts are embroiled in observing Corn Stunt and the corn leaf-hoppers, with 85 “sticky traps” from southwest, to south-central and northeast Kansas.
“We’re collecting (traps) every two weeks and sending them to Rodrigo (Onofre), seeing if they’re carrying both the corn leaf-hopper and the Corn Stunt disease,” Fanning said. “There are a few insecticides that can be used and some seed treatments that would control them. Some hybrids seem to handle it better than others. We’re still waiting on the seed corn companies to get us official data from South America.”
Some symptoms to Corn Stunt were different, such as putting on more ears that died prematurely and were “light as a feather,” he said. “For me, the tell-tale sign was those plants with three ears instead of just one normal ear.”
Tim Unruh can be reached at [email protected].
