Alfalfa is the primary forage crop in Wisconsin and is a key element in the state’s dairy industry. The yield of new varieties is greater than that of Vernal and other older varieties due to genetic gains made by breeders over the years. Much of the yield advantage of new varieties may be attributed to efforts to breed for resistance to a wide variety of major pathogens of alfalfa such as Verticillium, Phytophthora, and Aphanomyces. Although the yield gap has widened between new varieties and Vernal, yield of all varieties has declined steadily in Wisconsin during the past 30 years (Wiersma et al., 1997). There are several possible explanations for this situation including changing climate and the difficulties inherent in dealing with such a genetically diverse crop such as alfalfa. From a pathologist’s perspective, new disease-causing organisms or new strains of previously described pathogens may also play a role in limiting yield gains for Wisconsin alfalfa growers. At the University of Wisconsin-Madison, we are conducting research on alfalfa diseases, particularly those caused by organisms that have not been studied previously with respect to their presence and influence on the alfalfa crop. Aphanomyces root rot is one relatively new disease of alfalfa that has been studied in this regard.