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Pathogenicity of Fusarium spp. from Diseased Sugar Beets and Variation Among Sugar Beet Isolates of F. oxysporum. E. G. Ruppel, Research Plant Pathologist, U.S. Department of Agriculture, Agricultural Research Service, Crops Research Laboratory, Fort Collins, CO 80526. Plant Dis. 75:486-489. Accepted for publication 29 October 1990. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991. DOI: 10.1094/PD-75-0486.

Sugar beets (Beta vulgaris) with symptoms of Fusarium yellows were collected from fields in California, Colorado, Montana, Oregon, Texas, Wyoming, and Manitoba, Canada, during 1982–1988. Forty-eight isolates of seven species of Fusarium and two nonsporulating, unidentifiable “Roseum” types were obtained from root tissue on a medium selective for Fusarium spp. Three isolates of F. oxysporum (the reported causal agent of Fusarium yellows) from California, one each from Colorado and Texas, and two each from Montana, Oregon, and Canada, a Colorado isolate of F. acuminatum, two isolates of F. avenaceum from Texas, and one “Roseum” type from Colorado all caused foliar yellowing, wilt, root necrosis, and in some cases eventual death of sugar beet seedlings in greenhouse tests. Two isolates of F. solani (from California and Colorado) induced mild to moderate root rot of seedling taproots and secondary roots but no typical yellows symptoms. Of the isolates that were pathogenic to seedlings, only isolates of F. oxysporum and the Colorado isolate of F. acuminatum induced typical Fusarium yellows symptoms in wound-inoculated 3-mo-old sugar beets in the greenhouse. Other isolates pathogenic on seedlings induced some necrosis of secondary roots and arrested, necrotic lesions on the taproot of older sugar beets but no wilting, foliar yellowing, or plant death. All virulent and avirulent isolates were reisolated from surface-disinfested roots 30 days after planting in infested soil or 2 mo after inoculation of taproots. Isolates of F. oxysporum varied in growth, pigmentation, and conidial production on potato-dextrose agar. A significant isolate × cultivar disease interaction occurred when two sugar beet breeding lines, one susceptible and one resistant, were tested with four isolates of F. oxysporum from diverse geographic areas. In separate analyses of data from the resistant and susceptible breeding lines, the four isolates varied in virulence toward the susceptible but not the resistant line. Thus, the existence of physiological specialization among isolates of F. oxysporum from sugar beet remains an unresolved question.

Keyword(s): Fusarium equiseti, F. proliferatum, F. sambucinum.