Whip Corn Nematodes

January 5, 2011 06:39 AM
Whip Corn Nematodes

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."

Plants stunted by corn nematodes may appear to be suffering from a nutrient deficiency, she says. They are because the damaged roots can’t take up nutrients as they should.

"You need to know if nematodes are stealing yield," Bauer says. "Start by comparing roots from plants in a ‘hot spot’ to the roots of plants that look normal; see if they look different. Damaged roots may have a stopped-off or club-shaped appearance. The tips may turn brown, appear dead and stop growing; this can look like herbicide injury. Lack of root hairs also is a symptom of nematode damage."

Nematode testing. The only sure way to confirm the presence of corn nematodes is to send soil and root samples to a diagnostic laboratory. To find a laboratory, check with your land-grant university plant pathology department or your local Extension specialist.

It’s not enough simply to know nematodes are present; your laboratory analysis also will tell you which types and how many of each are in your fields. "A number of nematode species may attack corn," Bauer says.

"Some nematodes, such as sting, dagger, spiral, stunt, stubby root and ring nematodes, are ectoparasites, which means they feed outside the root. Lance and root lesion nematodes are endoparasites, feeding inside the root. Soil samples are effective at identifying ectoparasites. You also need root samples to find the endoparasites, the lance and lesion nematodes."

Bauer tries to collect soil and root samples four to six weeks after planting because some nematodes move deeper as the soil warms up in the summer. "If they get too deep, you won’t pick them up in your samples," she says. "Also, since the nematodes will be feeding in the root zone early in the season, you can take a shallower sample.

"Pull 10 soil cores per sample. Start 4" to 6" from the base of a plant and probe the soil at a 45° angle through the root zone.

"Before you head to the field, check with your lab about what depth of sample they prefer. Generally, if you sample at four to six weeks after planting, the lab will want a 6" to 8" depth. If you get into postharvest sampling, most labs want a 12" sample."

Along with soil samples, your lab will need a root sample. "Pull five root balls for each sample," Bauer says. "Dig up the entire root ball. Shake off most of the soil, and break off the plant just above the base of the crown."

Five root balls and 10 soil cores make one composite sample.

Store samples in sealable plastic bags to conserve moisture and keep the nematodes alive until the sample reaches the lab.

"If you will be sampling all day, store your samples in a cooler with ice packs," Bauer says. "If there will be a delay in shipping them to the lab, store the samples in a refrigerator. Ship samples early in the week, so they won’t lay around at the lab over a weekend."

Sampling strategies. Your sampling strategy will vary depending on whether you are analyzing a problem area, defining baseline levels of nematodes or comparing strip trials of nematode control products.

Yield maps and NDVI (Normalized Difference Vegetation Index) maps can help you identify problem areas, which you can ground-truth with soil and root samples.

To analyze a problem area, "don’t sample right in the heart of the bad area because after plants get in really bad shape, the nematode populations decrease and they move outward," Bauer says. "When this happens, you’ll see a circular pattern of damage, with the worst plants in the center. If you pull your sample from that area, you may miss the nematodes. Take one sample from the stunted area, outside the really bad area, and another from farther out, where plants have not been damaged. This is very important if your objective is to diagnose a hot spot."

If you’re not aware of any nematode problems, determining baseline levels will help you decide whether seed treatments will pay off in the future. "To determine baseline levels, you must respect soil types by pulling random soil cores from within each one," Bauer says.

In one field, Bauer found 180 lesion nematodes per root sample on higher ground, compared to 56 nematodes per root sample on lower ground. "If you mix different soil types in one sample, you’ll wind up with average readings, and you won’t be able to detect differences between risk levels," she says.

If necessary, you can pull samples after harvest; but sampling four to six weeks after planting, before nematodes go deep, is best. "If you sample at harvesttime, pull 12" soil cores, rather than 6" to 8"," Bauer advises.

Analyzing strip trials requires a systematic approach because nematode populations are highly variable.

"If half of your planter contained a nematode seed treatment and the other half didn’t, you can’t just pull one sample from each side," Bauer says. "Instead, pull a few replications within several small sample areas.

"For example, taking six samples out of the treated area and six from the untreated area will eliminate some of the variability," she says. "Take all the samples in a straight line, or transect, across the plots. Collect one of these multiple samples from an area where the corn looks bad and one from an area with less visual damage."

Collect the strip-trial samples—both soil and root samples—four to six weeks after planting.

Utilize test results. Base your control strategy on the treatment threshold for the species in your fields. "Treatment threshold for needle and sting nematodes, the most damaging species, in some cases may be as low as one nematode per 100 cc [about 1⁄2 cup] of soil," Bauer says.

Root lesion, lance, dagger and stubby root nematodes, which are considered moderate-risk, have higher treatment thresholds. Lower-risk species are spiral and stunt nematodes.

If you find a nematode problem, treatments are available, unlike a few years ago. With these new options, many growers are just one soil and root analysis away from higher corn yields.

Nematode Threat Is Real

A nematode sampling survey in southern Michigan, conducted by Farm Journal Associate Field Agronomist Missy Bauer (and funded by the Corn Marketing Program of Michigan) suggests corn nematodes are a bigger threat than previously suspected.

The June 2010 survey included 366 samples. Of those, 83% contained lesion nematodes in the soil or corn roots and 39% contained dagger nematodes, which are in the moderate-risk category. Only 2.5% had no nematodes.

The samples were assigned a risk index level from the Michigan State University Plant and Pest Diagnostic Laboratory. Only 40% of the samples were rated low- to no risk; 52% were found to be moderate-risk; 8% were rated high to severe.



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