While pest management can never be taken for granted, it certainly got easier with the advent of genetically engineered hybrids fortified with corn borer and rootworm resistance. But ironically, the technology may have helped a couple of other yield robbers become more of a threat. One, so far, is limited to northern areas of the U.S., and the other is present everywhere, just waiting for a chance to take a bite out of yield.
|Some entomologists believe that corn nematode populations are quietly increasing.
Nematodes are tiny nonsegmented worms. A few of the species that attack corn include corn needle nematode, lance nematode, lesion nematode and sting nematode. Most live in the soil and feed on corn roots from the outside, but some enter the roots and feed inside root tissue.
"Because nematodes exist in all soils, they are a potential problem wherever corn is grown," says University of Illinois plant pathologist Terry Niblack. "There are many species of nematodes, so various ones may become yield limiting in certain soils and localities."
Some species of corn nematodes cause no crop damage at all. Others cause a lot or only a little.
"In Kansas and Nebraska, sting and needle nematodes can wipe out every plant in some sections of a corn field," says Kansas State University nematologist Tim Todd. "Fortunately, those species occur only on our sandiest soils, and their damage is limited to patches.
"At the other end of the spectrum is the lesion nematode, which is found in every corn field regardless of soil type," Todd continues. "Their damage is in the form of chronic yield loss—a 5% loss is common across Kansas."
Inquiries about corn nematodes from farmers are increasing, Niblack says. A survey she conducted in northern Illinois revealed root lesion nematodes in every field, often in high enough numbers to damage yields.
Niblack, Todd and Michigan State University nematologist Fred Warner believe there are three reasons corn nematode populations could be climbing. First, because of genetically engineered corn hybrids, farmers are applying fewer soil insecticides. The older organophosphate and carbamate insecticides provided some nematode control. The newer pyrethroid insecticides do not. The other two reasons are more continuous corn and less tillage.
Symptoms of nematode damage can be mistaken for nutrient deficiencies, disease or other problems. The only way to find out is to have your soil analyzed. Niblack, Todd and Warner recommend monitoring fields for nematodes just as you do for fertility.
Monitor your fields. Iowa State University plant pathologist Greg Tylka recommends taking 10 to 20 soil cores, 8" to 12" deep, from every 20 acres. "Some laboratories also request two or three root balls, including the soil around the roots," he says. "If possible, collect a soil and root sample from a nearby area in the field that appears to be free of nematode damage so nematode numbers can be compared. Samples must be stored in a moisture-proof bag. They must be kept cool, but not frozen, until delivered to a laboratory."
- January 2009