If you find oval yellowing, stunted patches of alfalfa in your fields, you may have aphanomyces

“What I'm seeing all over the country, particularly on slopes, are yellow spots; 90% of the time that's going to be caused by aphanomyces,” warned Dan Undersander, extension forage specialist with the University of Wisconsin.

Undersander spoke recently at an America's Alfalfa/Forage Genetics day tour for seed representatives in West Salem, WI.

“What will happen is, the spot will stay a little shorter due to root pruning and then, if you come back a year or so later, oftentimes it'll start to fill in with weeds,” he added.

“So what can growers do now?” a seed dealer asked. “Well, nothing for existing stands,” Undersander answered. “Tell the farmer to plant an aphanomyces-resistant variety next time.”

He encouraged the crowd to watch for aphanomyces symptoms. “We're seeing tremendous amounts of it this year because we had a lot of wet springs. But the place to look is not the swales, but the slopes.”

If yellow patches are irregular and not oval, they may be pinpointing sulfur deficiencies, the forage specialist warned. “Watch for both on clients' fields,” he told the seed representatives, “particularly in the Midwest. We didn't put much fertilizer on this year because of the economy, and we're starting to pay for it.”

The key characteristic of aphanomyces is that the yellowing forms into oval shapes. In many cases, fields planted to aphanomyces race 1-resistant alfalfa will show such symptoms and need a switch to a race 2-resistant variety.

“We've been so successful with race 1 resistance that what's happening is the prevalence of race 2 is increasing rapidly. I've seen samples from New York to Idaho down to New Mexico that had race 2 in them.”

Alfalfa can be tested for aphanomyces, he says, but it's simpler and faster to clip the top 6” of plants and have them tissue-tested for sulfur deficiency. “If the sulfur and nutrient levels are adequate, then it's most likely aphanomyces.”

He also mentioned that varieties with race 2 aphanomyces resistance also are resistant to race 1. “So you never lose by planting a race 2-resistant variety.”