Alfalfa breeders are scrambling to find new sources of resistance to aphanomyces after the discovery last summer of infected fields that had a new race of the fungal disease.

The fields are in Minnesota and Wisconsin, but the extent and severity of the problem aren’t yet known, says Dan Undersander, University of Wisconsin Extension forage agronomist. Races 1 and 2 are prevalent across most of North America, and several other races are known to exist.

Fifteen or 16 races were identified by University of Wisconsin plant pathologist Craig Grau when he discovered aphanomyces in alfalfa more than 20 years ago, Undersander points out. Race 1 was most prevalent then; resistance to it was found in a plant from Afghanistan and breeders developed resistant varieties. Later, after race 2 became prevalent, breeders introduced varieties that warded off that one.
All varieties resistant to race 2 are also resistant to race 1 and a few additional races.

“We call it race 2 for simplification, but it’s actually about a five-race resistance,” he says. “So now some fields with alfalfa that has race 2 resistance are still showing aphanomyces. That means one of the other races, 6 to 15 or whatever, are coming up to cause a problem.”

If breeders can’t find new sources of resistance and the problem becomes widespread, fungicides are a possibility. Undersander and Grau have tested products that work, but they’re presently not cleared for use on alfalfa.

“They tend to be a little bit on the costly side, and that’s why we’ve stayed with plant disease resistance,” says Undersander. “But if that’s no longer an option, we might have to use some of these fungicides.”

Meanwhile, he tells growers to look for oval-shaped patches of yellowing, stunted alfalfa on field slopes. First have a tissue-test done to confirm or rule out sulfur deficiency, which is easily correctable.

If it’s not that, aphanomyces is probably the culprit, and yield will be reduced. If a significant portion of the field is affected, you might want to take the stand out early. Keep the field out of alfalfa for at least two years, then plant a race 2-resistant variety. If the current variety is race 2-resistant, plant a different one next time in case there are varietal differences, Undersander advises.