Submitted to: Review Article
Publication Type: Review Article
Publication Acceptance Date: 4/28/2015
Publication Date: 5/1/2015
Citation: Samac, D.A. 2015. Aphanomyces root rot of alfalfa: Widespread distribution of race 2 in Minnesota. Forage Quarterly. 2(2):12-13.
Technical Abstract: Strong seedling establishment in alfalfa is important to achieve the plant density needed to out-compete weeds and produce high biomass yields. Establishing alfalfa can be challenging because alfalfa seeds and seedlings are vulnerable to several pathogens present in soil. Wet soil conditions favor the development of diseases caused by “water molds,” namely Phytophthora root rot (PRR), Aphanomyces root rot (ARR), and damping off (rotting). When rain is excessive after sowing, stand establishment may fail due to seed and seedling rot caused by these diseases. Both ARR and PRR can also attack adult plants under wet soil conditions. To help protect seedlings from seed rot and damping-off and PRR, the majority of alfalfa seed is treated with the systemic fungicide mefenoxam (Apron XL®). However, Apron XL does not provide protection against ARR. Varieties with resistance to ARR are available, but the majority of varieties have resistance to only one race (race 1) of the pathogen. A recent survey of 45 alfalfa fields in Minnesota found that a second race (race 2) is widespread in the state and more common than race 1. Evidence is mounting that additional races of ARR are present in alfalfa fields that can overcome race 1+2 resistance. Recently, the fungicide Stamina, which protects seedlings against ARR, was labeled for use as an alfalfa seed treatment and is being used along with Apron XL to help boost protection from soilborne pathogens.