Like it or not, disease risk in no-till corn has to be managed. One solution is annual rotation with soybeans. But there are other options.
“Farmers need to pay more attention to disease risk in no-till compared to conventional tillage,” says Gary Munkvold, research coordinator for pathology and entomology at Pioneer Hi-Bred International, Johnston, IA.
Most growers aren't as aware of diseases as they should be, says Mark Carlton, extension crop specialist from southeastern Iowa.
Crop residues and diseases build up together in conservation tillage systems. In southeastern Iowa, for example, about 60% of land is seeded without tillage. As no-till acres increase, so do corn diseases.
Pathogens often start with small numbers late in the season and aren't a problem then. If they survive in residue, there are a host of pathogens ready to strike early the next season if environmental conditions are right.
Munkvold advises growers to scout for major diseases. Across the central Corn Belt, anthracnose and gray leaf spot are the two most likely to strike in mid-season or later. Some agronomists say it's best to check every field every week.
“A farmer may see spots on a plant and know disease is there, but it's a lot more difficult to determine the economic impact and whether it's something to be concerned about,” he says. “There isn't really a formula for answering that question.”
Generally, fungicide treatments in mid-to-late summer aren't an economical choice in corn. However, finding the problem early can help growers prevent bigger trouble next season.
Start by avoiding corn on corn in conservation tillage, if you can, Munkvold says.
“The longer you can rotate between corn crops, the lower the risk of disease,” he says. “The corn-soybean system isn't long enough to really eliminate disease problems, but it's certainly better than continuous corn or continuous soybeans.”
He adds, “If you do grow continuous corn, it's definitely wise to rotate among different hybrids, because each hybrid has strengths and weaknesses when it comes to diseases. If you keep planting the same hybrid in the same field, eventually it will lead to problems.”
For those who can alternate with soybeans, a one-year break from corn is “good enough,” says Carlton.
“If you plant corn into corn, any fungus that overwinters is right there,” he warns. “Plants are infected young, and the disease explodes.”
Results are vastly different, however, for growers who rotate to soybeans every second year.
“Some data show that disease-causing organisms live no longer than a year in the field. As the residue decomposes, they lose their food source and die off,” Carlton says.
Dean Malvick, University of Illinois-Urbana plant pathologist, agrees that under no-till conditions, corn on corn can boost disease problems.
“A two or more year rotation is a good solution. The pathogens are fairly common and can build up quickly,” says Malvick.
With new hybrids available every year, no-till growers need to keep an eye on genetic resistance traits.
Genetic resistance is the primary line of defense, according to Pioneer's Munkvold. When selecting seed, consider genetic resistance ratings as well as yield potential, he says.
“Our recommendation, as a company that specializes in crop genetics, is to carefully select a variety that has resistance to the diseases you encounter in the field,” he says. “Corn hybrids and soybean varieties differ in their resistance and success in high-residue situations.”
It can be hard to choose between yield and resistance. So in high-risk situations, put the emphasis on genetic resistance, Malvick advises. One high-risk situation might be a protected field in bottomland near a river; another might be a field adjacent to fields that had serious disease problems the past season.
“If you have two good hybrids with similar moderate resistance, go for yield,” he says.
Finely chopping corn residue will accelerate decomposition and may reduce disease levels. A stalk chopper can cut 18-in. stalks into 6-in. segments and leaves into shreds.
“If it's chopped fine, residue decomposes more quickly and pathogens won't survive as long,” Carlton says. “It won't eliminate a pathogen entirely, but it might reduce the amount of disease.”
But don't chop residue too fine; rain or strong winds may wash or blow it off the field.
Some growers use mechanical devices at seeding to clean crop trash off the seed row. This helps warm and dry the seedbed, but it won't reduce disease risk from airborne pathogens.
“The spores are either windblown or rain-splashed. If all you did was clean an 8-in. piece of ground, it would be highly ineffective for disease control,” Carlton says.
Most growers can achieve the benefits they want from conservation tillage without keeping all the residue on the surface, says Pioneer's Munkvold.
“By definition, you require 30-35% ground cover with crop residue to be considered a conservation tillage program. From a disease standpoint, it's not so important how you accomplish that. It's the overall level of residue that's going to determine the disease risk for the next crop you plant,” he says.
The number of spores produced is directly proportional to how much residue is on the surface, Munkvold explains. “If you cut it back one third, you'll significantly reduce the number of spores available to infect your crop. For gray leaf spot and other leaf diseases, you can significantly reduce the risk if you reduce the surface cover to 35% — and still leave significant amounts of residue on the surface.”
Strip-till and ridge-till are two tactics for this approach. Both reduce risks of root and seedling disease by improving soil conditions. “You're allowing soil around the seed to dry and warm up more quickly than it would under residue. That reduces the chances of those fungi attacking the roots,” he says.
Baling stalks or chopping silage to remove plant material from the field also should help, says Munkvold. “It should reduce the level of fungal contamination.”
As for residue distribution, he adds, “I don't think it's much of an issue for leaf diseases. The main thing, in relation to seedling and root diseases, is whether the residue is covering the row.”