Roots of nematode-damaged corn show fewer root hairs and stunted growth. Surveys show nematodes are a greater problem than previously suspected.
Corn nematodes may be the most neglected pest in your fields—and they could be
capping your yields.
"Very few farmers tell me they have a corn nematode problem," says Farm Journal Associate Field Agronomist Missy Bauer. "But we find nematode damage on many field calls. Often, it is misdiagnosed as something else, such as herbicide injury. Nematodes are kind of a hidden enemy."
Surveys confirm that corn nematodes are a growing problem. In a University of Nebraska survey that included 2,179 soil and root samples in 16 states, 81% of the samples contained lesion nematodes. "If you get high enough levels of lesion nematodes, you definitely will have yield loss," Bauer says.
The percentage of nematode infestation might have been even higher if the samples had been collected earlier in the season, before some species, such as sting and needle nematodes, burrowed deeper into the soil, Bauer notes.
Severe infestations of corn nematodes can reduce corn yield by as much as 100%. "I have
observed as high as 70% yield reductions in hot-spot areas with heavy pressure," Bauer says.
Corn nematodes include a number of species of microscopic roundworms, none of which are large enough to see with the naked eye. Compared to the better known soybean cyst nematodes (SCN), which at certain times of the year can just barely be seen, corn nematodes are much smaller.
Like SCN, corn nematodes live and feed on plant roots. "Corn nematodes are plant parasites, but they are not considered insects," Bauer explains. "At your land-grant university, you’ll find nematode information in the plant pathology department, not the entomology department."
Why more nematodes? Nematologists believe corn nematode populations are increasing for several reasons. "More corn on corn favors some species of nematodes that formerly were kept in check by rotating corn with soybeans," Bauer says. "Less tillage also favors nematode survival. The pyrethroid insecticides we now use have less effect on nematodes than the old organophosphate and carbamate products, which are being phased out. We also are applying fewer insecticides because of the use of transgenic corn."
No field is immune from the threat of corn nematodes. "It’s a misconception that nematodes occur only in sandy soil," Bauer explains. "Although some species live only in sandy soil, others occur in every soil texture. Even if you don’t farm sand, you still need to watch for nematodes."
Damage symptoms. Damage often shows up in circular areas or patches within a corn field. "The stand may look thinner in that area because corn is stunted," Bauer says. "When corn is in a rapid growth stage, damage may appear to dramatically worsen over a couple days’ time. In a situation like that, if you analyze the roots, you’ll find fewer root hair and stunted corn roots."
- January 2011