Alfalfa is an important crop for Wisconsin and the Midwestern, United States. Commodity records, as of February 1, 2013 report alfalfa prices reaching $265 per ton for hay (1). Nationwide, alfalfa was planted across 55.6 million acres in 2012, the 3rd field crop in terms of acreage after corn and soybean, and has an $8 billion dollar production value (15). Furthermore, alfalfa is the single largest source of protein for livestock, especially for the dairy industry (13). Wisconsin is the second largest producer of dairy in the United States, and since dairy feed is the single largest cost to the milk producer, the yield and consequent price of alfalfa is understandably important to the Wisconsin dairy industry (16).
Aphanomyces euteiches is a soil-borne oomycete that causes the disease, Aphanomyces root rot. A. euteiches can infect a variety of field crops worldwide, but in Wisconsin, the most important commodity is alfalfa. A. euteiches is most threatening in poorly drained soil conditions because it proliferates with water-motile zoospores. A. euteiches germinates in response to chemical signals from its host’s roots during early seeding, penetrates its host, and causes stunted, chlorotic hypocotyls and cotyledons due to necrosis of the roots early after emergence (12, 13). Although this disease does not cause immediate damping off, the pathogen stunts growth and reduces alfalfa’s ability to compete with weeds. This monocyclic oomycete is persistent and it is suspected that its oospores can survive as many as 30 years in soil that has not been planted with alfalfa. This suggests that A. euteiches can parasitize other hosts. Furthermore, A. euteiches has adapted to have increasingly more virulent phenotypes, beginning with race 1, race 2, and possibly now the most virulent race, race 3 (6, 12).
Currently, there exists no chemical treatment to manage A. euteiches infestations in alfalfa. The fungicide metalaxyl has been found ineffective against A. euteiches even though it effectively inhibits Phytophthora medicaginis, a second oomycete pathogen that frequently occurs in alfalfa fields (9). Farmers are left two management options for Aphanomyces root rot; crop rotations and planting with alfalfa cultivars that are selectively bred for resistance to specific races of A. euteiches. Currently, the commercial cultivar with the highest resistance available is only against race 2, which will be ineffective in prevention of A. euteiches of the putative race 3. Selectively breeding resistance to A. euteiches in alfalfa has aided in increased alfalfa yields; however breeding is a slow and costly process, especially since more virulent phenotypes than race 2 are predicted to exist (6). In addition, interest has peaked into using alfalfa varieties with the Roundup Ready trait. Anecdotal reports suggest that these Roundup Ready varieties lack the level of resistance to A. euteiches race 2 that exists in conventional varieties. This should be investigated further.