Effects of European and US strains of Fusarium spp. pathogenic to leafy spurge on North American grasses and cultivated species

Host-range tests were conducted in a greenhouse using 9 European and 11 U.S. strains of Fusarium spp. pathogenic to Euphorbia spp. Plants of 12 grass species native to the rangelands of North America were raised from seed, planted in soil infested with each strain, and assessed for dry weight after 24 weeks. Five of 11 U.S. strains of Fusarium spp. significantly reduced the dry weight of at least 1 species of grass native to North America. Only 3 native grass species were affected: Idaho fescue (Festuca idahoensis), big bluestem (Andropogon gerardi), and big bluegrass (Poa ampla). Mean reductions ranged from 56 to 92%. A single European strain caused a 53% reduction in dry weight of 1 grass species, sand love-grass (Eragrostis tichodes). Root-dip assays of 3-week-old seedlings in the greenhouse, with assessment over 3 weeks followed by 9 weeks of further observation and recording of dry weights of surviving plants mere used to assess pathogenicity to 27 cultivated plant species. Two of the 3 most virulent U.S. strains failed to cause disease on any crop species according to these criteria. Three U.S. strains were positive in root-dip assays, each to a single crop species, causing vascular discoloration of flax (Linum ussitatissimum) and root necrosis of okra (Hibiscus esculentus) and cotton (Gossypium hirsutum). Neither of the 2 most virulent European strains exhibited pathogenicity to any of 12 crop species. Two other European strains exhibited host ranges comprised of 3 and 4 crop species. Two strains of F. proliferatum from the U.S. and Europe differing in host range were vegetatively compatible. The greater frequency of disease incidence on Euphorbia in Europe and the narrow host range and apparently greater virulence of European strains indicate that strains well-adapted to leafy spurge might best include Fusarium spp. occurring in Europe.