Regardless of whether the production system is conventional or organic two of the most important constraints to snap bean production in the upper Midwest, are root rot disease and nitrogen availability, both of which may be physiologically and genetically related (Pfender and Hagedorn, 1982a; 1982b; Bliss and Miller, 1988). Snap bean is a legume that develops a symbiotic relationship with N2-fixing bacteria; nevertheless snap beans do not obtain adequate N from N2-fixation alone and supplemental fertilization is required for commercial production (LaRue and Patterson, 1981). The ability of snap bean roots to absorb nutrients from the rhizosphere is limited by rotting of lateral roots and infection of the vascular system of the root by several soil-borne pathogens, including Fusarium solani (Mart.) Appel & Wr. f. sp. phaseoli (Burk.) Snyd. & Hans, Rhizoctonia solani Kuenn, Pythium spp. and Aphanomyces euteiches f. ^.phaseoli (Yang and Hagedorn, 1966, Hoch et al., 1975; Pfender and Hagedorn, 1982a; 1982b; Kobriger and Hagedorn, 1984). In Wisconsin, the two most severe root rot pathogens are P. ultimum and A euteiches f. sp. phaseoli (Pfender and Hagedorn, 1982a; 1982b).
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