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Significance

Aphanomyces euteiches was identified in 1925 as the causal agent of a root rot disease that had plagued peas in Wisconsin for decades. Since its discovery, A. euteiches has been found throughout the U.S.A, Europe, Australia, New Zealand, and Japan. Initially, only pea was identified as a host of economic significance and ARR is still the most devastating disease of pea worldwide. On average, yearly losses to ARR are 10% although entire fields may succumb to the disease when environmental conditions are favorable for disease development. In the Great Lakes region of the U.S.A., pea production fields are intensively monitored for ARR potential and entire fields may be abandoned when inoculum levels reach a high ARR disease potential. Due to the longevity of oospores, fields with high ARR potential commonly remain unfit for pea production despite the absence of peas for years. The pea processing industry is typically located close to production fields to decrease the amount of time between harvest and processing. When fields are abandoned due to ARR, processing plants are closed and rebuilt near new production fields, often displacing workers and/or impacting local economies. The widespread occurrence of ARR in the Midwest has resulted in the pea processing industry decreasing pea production in the Midwest and eastern U.S.A. and increasing production in Idaho, Oregon and Washington where drier environmental conditions prevail. However, A. euteiches is common in the Pacific Northwest and can still cause significant yield losses in the region.

Many plant species were reported as hosts of A. euteiches prior to 1980. With the exception of pea, all were considered laboratory hosts and of no field relevance. A root disease of unknown etiology was observed on snap bean in Wisconsin in the 1970s. Symptoms were similar to several other root diseases, but different enough to indicate an unknown pathogen was present. Furthermore, methods of isolation from symptomatic tissues were inappropriate and delayed the identification of A. euteiches as the causal agent by Koch’s postulates until 1984.

Host specific forms of A. euteiches were determined to cause a root disease of alfalfa. For decades, the dogma was to avoid growing alfalfa in soils with poor drainage. However, the validity of this advice came to be seen in a new light with the discovery of Phytophthora medicaginis, the cause of Phytophthora root rot (PRR). PRR-resistant varieties were released in the early 1980s and alfalfa production was promoted for soils with drainage problems. Although PRR-resistant varieties provided a solution to unsatisfactory seedling establishment in many fields, poor seedling health and stand establishment remained an issue. By the mid 1980s, A. euteiches was confirmed as a significant pathogen of alfalfa, especially in conjunction with P. medicaginis. Since the initial studies in Wisconsin, A. euteiches has been identified as an economically important pathogen of alfalfa in other states. In Iowa, Kentucky, and Wisconsin, prevalence of A. euteiches in alfalfa fields often exceeds that of P. medicaginis. This observation may be partially explained by molecular studies of the interaction between A. euteiches and P. medicaginis within alfalfa roots, which suggest colonization by A. euteiches may reduce the colonization efficacy of P. medicaginis. Today, all modern alfalfa varieties are required to have resistance to both ARR and PRR to allow for successful seedling establishment and sustained forage yield.

Beginning in the early 1990s, variation in virulence was observed within populations of A. euteiches isolated from alfalfa. Isolates of A. euteiches virulent on alfalfa cultivars susceptible to ARR but avirulent on alfalfa cultivar WAPH-1, the hitherto source of resistance to ARR, were designated race 1. Isolates virulent to both susceptible alfalfa cultivars and race-1-resistant WAPH-1 were designated race 2. Race 2 isolates of A. euteiches are now prevalent in many alfalfa-producing states. Resistance to race 2 has been identified, which provides a yield advantage over ARR-susceptible cultivars and race 1-resistant WAPH-1, and race 2 resistant cultivars are now available to growers.

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