There's a new “race” to keep alfalfas free from a fungal disease that has been called one of the U.S.'s major limiting alfalfa diseases on poorly drained soil.
Actually, the race is being made by alfalfa companies — to provide resistance to another devious form of aphanomyces root rot.
“A number of new alfalfa varieties are coming on the market that are resistant to race 2 aphanomyces,” says Dan Undersander, University of Wisconsin extension forage specialist.
About 15 years ago, Undersander says, aphanomyces was identified as a fungal disease that can infect, stunt and cause alfalfa seedlings to yellow before they wilt and die. It differs from phytophthora root rot and pythium, which tend to kill seedlings quickly before they turn severely yellow.
Aphanomyces also causes yellowing and stunting in established stands — and reduces yield, he says.
“If you planted an alfalfa variety that is highly resistant to phytophthora and still see the yellowish stunting, the field is likely infected with aphanomyces. It may also increase winter/spring root heaving damage to alfalfa.”
Although alfalfa companies have bred aphanomyces resistance into their varieties over the years, they have recently had to work to develop resistance to a second race.
Race 2 aphanomyces is probably geographically widespread over southwestern and central Wisconsin, Iowa, Kentucky and the East Coast. It occurs in combination with race 1, Undersander says.
“Growers with poorly drained soils should consider planting race 2-resistant varieties,” he advises. “These varieties are also resistant to race 1 aphanomyces.”
Check Hay & Forage Grower's 2004 Alfalfa Variety Guide, starting on page 13 of this issue, for the column indicating which varieties offer race 2 aphanomyces resistance.