DISEASES OF ALFALFA - SEEDLING BLIGHTS

Another way to identify whether seedling problems are caused by Phytophthora medicaginis or Aphanomyces euteiches is to check about the alfalfa variety that has been planted. Many commercially available alfalfa varieties have been bred for resistance to one or both of these pathogens. If the grower has planted a Phytophthora-resistant variety and still is having seedling problems, then A. euteiches may be the problem. Conversely, if the grower has planted an Aphanomyces-resistant variety and is still having seedling problems, then P. medicaginis may be the problem.

Aphanomyces root rot-resistant alfalfa varieties are “race specific”. This means that the resistance is effective against some, but not all isolates of A. euteiches. Therefore a grower may want to have soil tested prior to planting. Testing will help you determine whether P. medicaginis or A. euteiches is present, as well as help identify which race(s) of A. euteiches. Based on this information a grower will have a better sense of which alfalfa varieties are most likely to be effective for disease control in a field.

In addition to asking about varieties and their disease resistance, also ask whether a grower has applied any seed treatments prior to planting. Seed treatments containing metalaxyl or mefanoxam can provide control of P. medicaginis, but will not control A. euteiches. Be aware however, that extensive use of metalaxyl/mefanoxam has led to development of some strains of certain Phytophthora species that are no longer effectively controlled by these products.

In this slide seeds on the left were treated with Ridomil (a metalaxyl/mefenoxam-containing product), those on the right were not treated. Because the non-treated plants are smaller and less thrifty, this suggests that P. medicaginis is present in the soil at this site.