Your alfalfa may be suffering from a root lesion nematode infestation if you have trouble:
* Getting a stand established, even after seeding a field two or three times.
* Holding your stands longer than two or three years, yet you're not able to single out any of the traditional diseases that can limit stand life.
Root lesion nematodes feed on alfalfa's root hairs, points out Neal Martin, University of Minnesota extension forage agronomist. Of course, the root hairs are where the nodules that fix nitrogen are located. So finding no nodules can be an indicator of the presence of lesion nematodes.
"Since the alfalfa plant has lowered vigor due to less nitrogen available and potential feeding on the crown, you'll see the damage as the thinning out of two-year-old stands," says Martin.
The reason you haven't heard more about the nematodes is because not enough diagnostic testing has been done, he believes.
"I think root lesion nematodes are a problem that a lot of farmers are having some trouble with in the Upper Midwest, yet they're not really aware of it," says Dan Undersander, University of Wisconsin extension forage specialist. "Instead, they attribute not being able to get a stand to bad weather or something else. But if they have a soil sample analyzed, generally, sure enough, there are high levels of the root lesion nematode.
"We really don't know the extent of the problem at this time," he adds.
Root lesion nematode hot spots have been identified in Minnesota, Wisconsin, Michigan, Kansas, Iowa, Illinois and New York. The likelihood of infestation apparently is higher in less thrifty fields.
"It's usually the lighter soil types rather than the heavy clays where we see a root lesion nematode problem," advises Deborah Samac, a research plant pathologist at USDA-ARS. "Often, too, the situation is associated with fields that were previously in pasture or CRP."
She points out that, as other diseases and cultural problems have been eliminated, nematode problems in general have become more apparent.
Although not every grower has the tiny pests in his fields, heavy damage has been found throughout the Midwest, says Samac.
Secondary hosts that don't suffer the effects of lesion nematodes are numerous, including quackgrass, oats, white cockle, and dandelion. Seeding alfalfa after alfalfa apparently adds to the problem.
Only two common crops - ryegrass and sorghum - are not hosts. They would be the crops to grow before alfalfa in problem fields, says Undersander.
"Although root lesion nematodes have been around for years, we've just newly discovered them as a reason for stand failures," he continues. "And I think they're gradually getting worse."
Undersander says that they can be spread by tillage implements moving from infected fields to uninfected fields.
"If you don't pay attention they will gradually spread."
Only two resistant varieties are available: WL252HQ and newly released Dagger+EV.
The next-best strategy, according to Martin, "is to look for a variety that's performed well at the North Central Research Station at Grand Rapids, MN, because there's always nematode pressure at that location."
"There may be some resistant varieties that aren't rated as resistant," agrees Mike Peterson, WL Research.
Complicating the situation, Peterson adds, is that there's a complex of four to eight nematode species. And variety resistance to root lesion nematodes doesn't necessarily carry over to the other species.