The Race to Beat Race 2

Mike Rankin
Crops and Soils Agent - Fond du Lac County
University of Wisconsin - Extension

Several years ago, the Fond du Lac Forage Council sponsored a project to quantify the presence of Aphanomyces in area soils.  Recall that Aphanomyces root rot is a potentially destructive disease during and after the establishment phase of alfalfa.  The study effectively confirmed that the disease is present in almost all area soil types. Unlike several years ago, there are now many alfalfa varieties with R or HR ratings for Aphanomyces.  Recently, one of the alfalfa breeding companies had an extensive article on a race 2 Aphanomyces pathogen in an issue of their newsletter.  It pointed out that alfalfa breeders are currently in the process of developing varieties resistant to this strain.  This begs the question, "How concerned should Fond du Lac County alfalfa producers be with race 2 Aphanomyces?"

According to Dr. Craig Grau, Extension Plant Pathologist, the alfalfa Aphanomyces population is composed of two virulence types; race 1 and race 2. Race 1 forms do not cause severe disease on commercial varieties with an R or HR rating.   Race 2 isolates cause severe disease on Race 1 resistant varieties in lab tests; and early field experiments indicate Race 2 resistant breeding lines out-yield race 1 resistant varieties if Race 2 is predominant.

For some reason the first isolates of Aphanomyces encountered would be called race 1 today.  However, around 1990 UW researchers recovered isolates that were highly virulent to breeding lines resistant to Wisconsin isolates.  The highly virulent isolates were recovered from soils collected in eastern and southern states.  Grau and his co-workers initiated a wider search in Wisconsin and found a few what would be called race 2 isolates today.  With one exception, these race 2 isolates were not widespread. The one exception were southwestern counties where the incidence was much higher.  Grau points out that the Lancaster Research Station in Grant County is highly infested with race 2.  Many samples from Eastern Iowa also have high levels of race 2.  It’s possible that race 2 predominance has something to do with non-glaciated soils but researchers don’t know this for sure.

Most companies are working hard to develop alfalfa varieties with high levels of race 2 resistance.  The issue of need for race 2 resistant varieties will surely come-up when such varieties hit the market and become a marketing strategy.  Fields that continue to have annual or periodic establishment problems are candidates to investigate the value of Race 2 resistance.

Does planting fields to race 1 resistant varieties result in selection pressure for race 2 to emerge as the predominant form of Aphanomyces?   Low levels of race 2 have always been present at research sites on the Marshfield Research Station.  Race 1 resistant material has been planted in the nurseries since 1988.  However, race 1 resistance appears to remain effective even though race 2 isolates are becoming less difficult to detect.  The good news is that breeding lines with high levels of resistance to race 2 express super high levels of resistance to race 1 isolates.  Grau’s guess is that someday all varieties will have race 2 resistance and Aphanomyces will be under even greater genetic control.

For more information contact Mike Rankin