First report of fusarium wilt of alfalfa caused by fusarium oxysporum f. Sp. medicaginis in Wisconsin

Fusarium wilt is an economically important vascular disease of alfalfa (Medicago sativa) throughout the world (Rhodes 2015). During the summers of 2013 to 2016, alfalfa plants with foliar wilt symptoms and reddish-brown discoloration in the root stele and basal stem, consistent with symptoms of Fusarium wilt (Rhodes 2015), were observed across disease nursery field plots with multiple germplasms near Arlington, Wisconsin. Symptomatic roots were dipped in 70% ethanol, flame sterilized, cut into thin slices, and incubated on potato dextrose agar medium amended with 0.05% streptomycin. Colonies of white mycelia with tan sporodochia producing macro- and microconidia morphologically similar to Fusarium spp. were observed after 3 weeks of incubation at room temperature. Six single spore isolates were characterized using spore morphology, diagnostic DNA sequences, and a pathogenicity assay. Macroconidia were hyaline, falcate, had 3 to 5 septa, and measured 25 to 45 x 6 μm. Microconidia were hyaline, oval, non-se...