Host species specificity of the goldenrod rust fungus and the existence of rust resistance within some goldenrod species

Abstract

Five out of six species of goldenrod at a study site in Ontario had rust pustules in the field and also developed pustules after laboratory inoculation with urediospores of a single-pustule isolate of the rust fungus (Coleosporium asterum) obtained from Solidago canadensis. This isolate also successfully infected seedlings of S. canadensis and an additional goldenrod species from British Columbia. Light microscopy revealed low levels of spore germination and high levels of failed infection attempts even on susceptible plants; in S. canadensis, infection commonly failed before the formation of the first haustorium, a phenomenon typical of nonhost resistance and indicative of a degree of nonspecific incompatibility between fungus and host. Only 2 out of over 200 S. canadensis seedlings from Ontario or British Columbia lacked pustules after repeated inoculations compared with 7 out of 36 for Solidago rugosa seedlings from the study site. Microscopically, infection sites in most of these resistant plants resembled those in crop plants with parasite-specific, single-gene resistance. The data raise the possibility that the fungus has exerted a stronger selective pressure for parasite-specific resistance on S. rugosa than on S. canadensis. Key words: Coleosporium asterum, goldenrod, rust fungi, Solidago, specificity.