Midwestern alfalfa growers should be on the lookout for yet another new race of the fungal disease aphanomyces, advises Dan Undersander, University of Wisconsin Extension forage agronomist.

“We’re finding that there are some areas, particularly in southwestern Wisconsin and northern Illinois and into Iowa a little bit, that are getting aphanomyces even when we plant race 2-resistant varieties. So that means that we now have sorted out that we have another race that is becoming important,” says Undersander.

Some producers may still be catching up with the emergence of race 2 in 2009, he says. “We’re actually in the process of promoting race 2 resistance.  A lot of areas are just shifting from race 1.” Growers should monitor for telltale oval-shaped patches of yellowing, stunted alfalfa often associated with aphanomyces.

Aphanomyces may not always be the culprit of those yellow patches, however. Undersander recommends that growers tissue-test to identify possible sulfur deficiencies first. If they’re ruled out, aphanomyces, which stunts plants and reduces yields, is likely to blame.

Because breeding and seed increases are slow processes, Undersander says, bringing another race-resistant variety to market could take several years. “And the question is, can we find a source of resistance?” he adds.

For growers looking for more immediate options, Undersander advises paying close attention to soil fertility.

“Some of the areas that we’re seeing this race 3 resistance in, we’re also very commonly seeing sulfur deficiency. So the main thing at this point would be to plant a race 2-resistant variety, make sure all nutrients are at optimum levels, and, in particular, pay attention to potassium and sulfur.”

If a field already shows a lot of aphanomyces damage, Undersander recommends rotating it to another crop for a couple of years and then seeding it back with a race 2-resistant variety.

Aphanomyces seems to prefer wet conditions during establishment, he cautions. Under those conditions, he advises growers to encourage rapid growth by planting at the correct date, having good soil fertility and pH and controlling weeds during the first 60 days. “If we can get that plant up and get it growing fast, it’ll be less likely to catch aphanomyces.”