HOST-RANGE EXPANSION OF THE ALFALFA RUST PATHOGEN

A systematic investigation of the host range of a monouredinial isolate (KR1-1) of Uromyces striatus from alfalfa (Medicago sativa) was undertaken. The extent of susceptibility to this pathogen within plant tribes closely related to alfalfa was determined. A total of 844 plant introductions, representing 345 species or subspecies from 27 genera, was tested. The plants tested included representative species from the tribe Trifolieae (which includes alfalfa) and seven additional tribes closely related according to current phylogenetic descriptions of the Leguminosae. Six of the eight tribes contained genera with susceptible species that supported urediniospore production. A total of 141 species or subspecies from 11 genera was susceptible to KR1-1. Susceptible plants from the tribes Trifolieae, Cicereae, and Vicieae generally supported profuse urediniospore production, whereas susceptible species in the tribes Genisteae, Galegeae, and Hedysareae supported sparse urediniospore production. No susceptible species were found in the tribes Coronilleae or Loteae. The distribution of degree of susceptibility followed the current proposed phylogenetic relationships of these tribes. These results indicated that an isolate of U. striatus, found as a parasite of alfalfa, is capable of surviving and reproducing on a broad range of plant species. Parasitic efficiency was decreased with increased phylogenetic distance of the host species from alfalfa. The extent of the host range probably is a significant factor in the epidemiology of this alfalfa pathogen.