Poor growth of seedling alfalfa in wet or slowly drained fields may be due to several diseases. Aphanomyces root rot causes death and stunting of seedlings as well as more subtle disease of established plants that can result in significant yield reduction. Other diseases that occur in wet or poorly drained soils include Phytophthora root rot and Pythium seed and root rot.|
Plants infected with Aphanomyces usually become stunted and chlorotic (yellow) before they wilt and die, whereas Phytophthora and Pythium tend to kill seedlings quickly before plants become severely chlorotic. Another clue to a problem with Aphanomyces is root rot of an alfalfa cultivar that is highly resistant to Phytophthora.
Although not much is known about Aphanomyces root rot in Illinois, it is known to be a serious problem in nearby states including Wisconsin, Iowa, and Kentucky. We suspect this disease is also a problem in many Illinois fields. This disease is caused by the soilborne fungal-like pathogen Aphanomyces euteiches. Aphano-myces root rot has been recognized as a serious disease of processing pea for almost 80 years. Perhaps because alfalfa disease that occurred in wet soil was attributed to Phytophthora, Aphanomyces root rot of alfalfa was not recognized as a serious problem until the early 1980s.
Aphanomyces root rot can best be managed by avoiding poorly drained soils and using Aphanomyces-resistant alfalfa varieties. Fungicides are not available for control of Aphanomyces root rot of alfalfa. Phytophthora and Pythium root rots of seedlings can be controlled with fungicidal seed treatments, such as Allegiance-FL, ApronXL, or Apron-FL, but these seed treatments are not effective against Aphanomyces.
Alfalfa varieties rated highly resistant (HR) or resistant (R) to Aphanomyces root rot should be planted where slowly drained soils occur and where Aphanomyces may be a problem. Control of Aphanomyces root rot became more challenging when different races of this pathogen were discovered. Many commercial alfalfa cultivars are now available that have resistance to race 1, the first race discovered. Another race (race 2) of Aphanomyces was identified in the early 1990s that overcomes race 1 resistance. Alfalfa cultivars developed for resistance to race 1 are killed by the aggressive race 2 isolates. Race 2 isolates have been identified in a number of states including Wisconsin, Iowa, and Kentucky. Race 2 has not yet been confirmed in Illinois. Alfalfa varieties with resistance only to race 1 may be genetically vulnerable to Aphanomyces root rot in many regions due to the presence of race 2. Several commercial alfalfa varieties are now available that have resistance to both races of Aphanomyces. If resistance to race 2 is not specified for an Aphanomyces-resistant alfalfa cultivar, then you can assume it is resistant only to race 1. The overall distribution and impact of races 1 and 2 of Aphanomyces are uncertain, but Aphanomyces root rot should be considered as a potential problem in many parts of Illinois.--Dean Malvick