By ANN HINCH
Assistant Editor URBANA, Ill. — Nematodes.
It’s a word guaranteed to strike worry into the hearts of soybean growers. The soybean cyst nematode (SCN) is seemingly always waiting to feed on the valuable plants, and farmers want to be vigilant about spotting signs of early damage.
But there are nematodes that attack corn, too. Agriculture hasn’t paid them much notice for a couple of decades, explained Dr. Tamra Jackson, but that’s changing as everybody scrambles to learn more about the microscopic worms, since so much more attention is being paid to corn production now.
“When corn prices get that high, our producers are not willing to let anything knock off that corn if they can help it,” said Jackson, with the University of Nebraska-Lincoln Department of Plant Pathology, addressing a group at last week’s annual AgMasters Conference at the University of Illinois.
Scientists know more about SCN at this point than about nematodes of corn (she was careful to point out “corn nematodes” is a misnomer since most will feed on more than corn), but she said it is suspected corn feeding is increasing partly because of different production practices than 20 years ago.
For one, farmers have transitioned away from old insecticides in favor of less environmentally toxic substances; Jackson said the old ones had “secondary nematodal activity” that killed the little worms. Also, growers use less insecticide in favor of planting Bt corn that resists cutworms. She said this transgenic trait does its job, but like the newer insecticides, hasn’t had much effect on the nematodes.
There are new seed treatments available, though, that are specifically nematicides. Jackson explained these temporarily coat the seed and give its root system “a head start” into maturity. These include Avicta from Syngenta, Poncho VOTiVO from Bayer CropScience and Acceleron from Monsanto.
Weed management is also helpful, since some of these are just as good a target for nematodes to munch on as corn. Basically, she said anything that lessens stress for a young corn plant will make it better able to resist nematode attacks if it is damaged later, as from high winds or hail, since nematodes find their entry into plants through wounds.
She said less crop rotation – monoculture – also contributes to nematode breeding. In the wild prairies that boast many different kinds of plant life, native U.S. nematode species don’t get as much traction because the plant variety makes it harder for them to find food. Tilling also spreads them out, she added.
“Anything that moves soil moves nematodes, and other pathogens that live in soil, for that matter,” Jackson said.
There are several kinds of nematodes that feed on corn; and within each genus are several species of the tiny worm. Their genus names are pretty descriptive of their appearance or function – a needle nematode is perhaps the largest and can grow up to nearly 1/8-inch long, is more than 10 times the size of SCN, and is long and thin.
There are also sting, dagger, lance, root-knot and root-lesion, to name a few. Finally, not all nematodes are damaging – some types are beneficial and eat the more crop-harmful varieties.
Misconceptions One misconception Jackson has encountered about nematodes of corn is that not all of them live in sandy soil. “Some nematodes don’t give a hoot about soil texture,” she said, pointing out sandy soil is easier for the larger ones to move around within.
Another misconception concerns the appearance of damage to plants. Jackson showed a slide of a field of lush, green plants that, when tested, showed more than 10,000 nematode eggs per cup of soil. That, she said, is very high.
Obvious signs of damage include yellowing, root necrosis, plant stunting and uneven height and stubby roots. Just because a field shows these symptoms, however, does not mean nematodes are doing it. Many things cause similar damage, such as insects, herbicide injury and soil pH imbalances; she cited one Nebraska field that looked to be at the mercy of nematodes when, in fact, the plants were suffering aluminum toxicity from low-pH soil.
Needle and sting nematodes cause the most damage if they take hold in a cornfield; fortunately for growers, these are also the most uncommon kinds found. On the other end of the scale is the spiral nematode, which has been tested at 3,000-4,000 per gram of dry root in south-central Nebraska cornfields that produced yields of 250 bushels per acre. A couple of other less dangerous nematode types are the stunt and dagger.
For Midwest corn acres, perhaps the most important type of nematode is the root-lesion, a moderate damage risk. In a Syngenta-sponsored survey of Nebraska cornfields in 2006, Jackson said root-lesion nematodes were found in 81 percent of locations sampled. Further, many types of plant-parasitic nematodes were found in more than 99 percent of the samples.
“We could find nematodes out here in the lawn, that could feed on corn,” she said.
Lab testing tips Nematodes feed on plants through vulnerable wounds, and some of them crawl inside the roots to live. Jackson said this is why it’s important that if one is going to submit soil samples to a lab for nematode testing, it’s also a good idea to include root samples if possible – but check with the lab to see if it will perform root testing and whether it requires a whole root ball or can work with fragments.
When collecting soil samples for a lab, growers should sample the perimeter of a problem spot; for example, if the field has an oval-shaped area of plant death/damage, take a sample from the edge of it, not in the very center (it is also good to get a separate sample from a healthy-corn perimeter, for comparison).
About two cups of each soil is best, double-bagged in “lock” plastic bags and shipped overnight to the lab if possible. Until shipping, keep cool but don’t freeze – a refrigerator is ideal.
Sampling more than once in a season is a good idea, too. To get a baseline for soil, test it right before planting and again 4-8 weeks after planting, 4-8 inches from the plants in the same location, in the upper 8-10 inches of soil. This will particularly help in deciding whether to use seed treatments the following growing season.
Other questions to ask a lab are if personnel are trained to count nematodes in samples, since Jackson said this is a more specialized skill than simply looking for the presence of nematodes. In her lab, only she and one other person can do this counting.
Also, since labs are unable to accept samples from out-of-state without federal permits, some may not, or charge more to do so.
Nematode eggs can overwinter in soil or old roots; even the adult parasites can rest in a quiescent state to overwinter, similar to hibernation in some mammals. They are adaptable and can change their cycles to correspond to the growing of their favorite foods.
“What makes a corn plant happy makes a nematode happy, too,” Jackson said of soil conditions.
To learn more about test results and nematode research, visit UNL’s Plant Disease Central website online at http://pdc.unl.edu |
