REEXAMINATION OF RACES OF THE CUCURBIT ANTHRACNOSE PATHOGEN COLLETOTRICHUM-ORBICULARE

Seven physiological races of the cucurbit anthracnose pathogen Collelo trichum orbiculare have been previously described on the basis of dise ase reactions on 12 differential cucurbit hosts. In this study, 89 iso lates of C orbiculare (35 from cucumber, 33 from watermelon, two from cantaloupe, four from cucuzzi gourd, two from honeydew, 10 from cockle bur, and three from unknown cucurbit hosts) and three isolates of C ma gna (two from watermelon and one from acorn squash) were examined for vegetative compatibility with the use of nitrate nonutilizing mutants. The collection included a representative culture of each of the seven previously described races obtained from the American Type Culture Co llection. Twenty-eight isolates, selected to represent all vegetative compatibility groups (VCGs) and several previously identified races, w ere examined in greenhouse cotyledon inoculation assays on 13 cucurbit hosts. Ten VCGs were identified among all isolates examined; however, only isolates in three VCGs (VCGs 1, 2, and 3) were pathogenic on the cucurbit differentials in greenhouse virulence tests. VCG 1 was compo sed of 28 cucumber and two cantaloupe isolates, and VCG 2 was composed of 33 watermelon, two cucuzzi gourd isolates, and a single isolate wi th an unknown host origin. Seven older cucumber isolates, collected pr ior to 1986, belonged to a third VCG (VCG 3). Of the three VCGs pathog enic on the cucurbit differentials, two virulence phenotypes could be identified. Isolates in VCG 1 gave disease reactions similar to isolat es previously described as race 1, whereas isolates in VCG 2 gave dise ase reactions similar to isolates previously described as race 2. The cucumber isolates in VCG 3 also gave disease reactions typical of race 1-type disease reactions. The C. orbiculare population pathogenic on cucurbits in the United States appears to have a limited VCG diversity . Within this population, there was a distinct correspondence between host origin, VCG, and virulence (race) phenotype.