For years, Upper Midwestern growers and plant pathologists blamed winter weather for damaging what looked to be perfectly healthy alfalfa plants the fall before.
Maybe they should have accused brown root rot (BRR) of the dirty deed, suggests Craig Grau, University of Wisconsin plant pathologist.
A fungal disease, BRR can cause the same symptoms as winter damage or several other root rots, says Grau. Infected alfalfa roots that look a healthy white going into winter can rot by spring, causing slight injury to severe damage and death.
“It's primarily a problem in older stands,” Grau says. “While most of our fungi are active during spring and summer, this one is asleep. When the plant goes into dormancy and temperatures drop, it starts its activity. These are fungi that can grow below 32°F.”
Certain winter conditions seem to trigger BRR more than others, he says. “The greater the snowfall, the greater the damage. So years when we can't explain winter death — because we have nice snow cover — I think those are brown root rot years.”
Sometimes BRR doesn't show symptoms, says Deborah Samac, a USDA-ARS research plant pathologist based at the University of Minnesota.
Samac has spent much of the past year testing for BRR in alfalfa plants sent mostly from around Wisconsin and Minnesota. She uses a recently developed molecular test that identifies BRR by its DNA.
“The samples I received in the fall didn't have the classical symptoms,” she reports. “It was only during the spring that I saw root rot symptoms.
“Brown root rot looks fairly widespread in Minnesota, but this is based on a preliminary survey,” says Samac.
Grau adds that the disease has been found as far east as New York and as far west as Idaho. “It's probably going to be found mainly in this upper tier of the lower 48 states. In the Midwest, it hasn't been found south of the Minnesota-Iowa line.”
Other than knowing when and where BRR hits, Samac wants to find out at what thresholds it harms alfalfa. She also wants to study what happens to the fungal population once a non-host crop rotates onto an old alfalfa field. Finding out which varieties hold up better against BRR is another goal on Samac's list.
A state grant will allow her to research those questions. She also hopes to develop a soil test to determine if the fungus is in a field and an easier DNA sampling method to tell if plants are infected.
In the meantime, Grau suggests that growers rotate out of alfalfa for at least two years to a crop other than a forage legume. “There is more evidence that — not only for brown root rot but maybe for other problems as well — alfalfa producers need to put a few years of some other crop between the year they take out an alfalfa stand and the year they re-establish.”
Producers with possible BRR problems should also choose varieties with high winter survival scores and high resistance to Pytophthora, Aphanomyces and other root diseases.
Longer-term, BRR-resistant varieties are needed. Grau and Samac say the seed industry is, at this time, concerned and has supported the recent survey work.
A BRR-resistant variety is in the works at the University of Wyoming, but it probably won't be right for the Midwest, says Samac. “It's probably a little bit more dormant than our growers would like. It also may not have some of the disease resistance that we need here, like bacterial wilt or Fusarium wilt.”
BRR-Resistant Variety Developed
Growers in Wyoming and surrounding states can soon look for a new brown root rot-resistant variety from the University of Wyoming.
The variety, currently called WY-BRRR, is being tested in Wyoming and Montana trials. Foundation seed is being produced and certified seed should be available to growers in 2006, according to Fred Gray, plant pathologist at the University of Wyoming.
WY-BRRR is comparable in brown root rot resistance to Peace, a Canadian variety. But it should produce higher yields than Peace, Gray adds. “Our new variety should also exhibit good long-term persistence because the parental materials survived six winters in a harsh Wyoming environment.”