Improving Our Understanding of Aphanomyces Root Rot of Alfalfa

Aphanomyces root rot of alfalfa, caused by the oomycete pathogen Aphanomyces euteiches, is an important disease of alfalfa in poorly]drained soils in Kentucky. The disease can reduce alfalfafs growth and disease]stressed plants are unable to out]compete weeds, leading to thin, weedy stands. Aphanomyces euteiches survives in soil for many years, even when alfalfa is not planted, meaning that once the disease is present, alfalfa will always need to be managed for Aphanomyces root rot. In addition to the longevity of the pathogen, A. euteiches can evolve and adapt to become more aggressive and damaging, and the development of these more aggressive gracesh has impacted alfalfa production in Kentucky. Currently, A. euteiches race 1 and race 2 are present in Kentucky. However, the distribution and frequency of these races is not well]known since the last comprehensive survey for A. euteiches was conducted from 1990]1992 (Vincelli et al., 1994). One sample from Lexington was race]typed in 2001 and determined to be race 2 (Malvick and Grau, 2001), and it is assumed that race 2 is widely prevalent in Kentucky, although there is little public research to confirm this. Aphanomyces root rot management is primarily achieved by planting cultivars that have racespecific resistance. However, in spring of 2019, Aphanomyces root rot was reported in fields planted to cultivars with race 2 resistance. This has led to concerns that a new race is developing (race 3) or potentially a more aggressive form of race 2. This information is critical to understand, since if there is a race 3, or adapted race 2, there will be very limited management options available for farmers. New chemical seed treatment active ingredients such as pyraclostrobin and tolclofos]methyl are now labeled for A. eutieches, however their activity against A. eutieches has not be tested in public trials in Kentucky. These treatments have been shown to have some efficacy in other states (Jensen et al., 2018), but climate and population differences may impact efficacy in Kentucky. Improving management of Aphanomyces root rot in Kentucky starts with understanding the population and race structure of the pathogen, and its ability to cause disease on resistant cultivars. Seed treatment management should also be investigated to determine if multiple management strategies can help alleviate the effects of disease until race structure and improved cultivars are available. The objectives of this proposal are: 1) Soil sample alfalfa fields in Kentucky and determine race structure of A. eutieches using a race bio]assay; 2) Determine if new chemical active ingredients have efficacy against A. eutieches in field research trials in Kentucky; and 3) Distribute management information on Aphanomyces root rot in diverse formats to Kentucky alfalfa farmers.