VIEW ARTICLE

Genetics

Reexamination of Races of the Cucurbit Anthracnose Pathogen Colletotrichum orbiculare. L. A. Wasilwa, Graduate research assistant, Department of Plant Pathology, University of Arkansas, Fayetteville 72701; J. C. Correll(2), T. E. Morelock(3), and R. E. McNew(4). (2)associate professor, Department of Plant Pathology, University of Arkansas, Fayetteville 72701; (3)professor, Department of Horticulture and Forestry, University of Arkansas, Fayetteville 72701; (4)professor and department head, Department of Agricultural Statistics, University of Arkansas, Fayetteville 72701. Phytopathology 83:1190-1198. Accepted for publication 14 July 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-1190.

Seven physiological races of the cucurbit anthracnose pathogen Colletotrichum orbiculare have been previously described on the basis of disease reactions on 12 differential cucurbit hosts. In this study, 89 isolates of C. orbiculare (35 from cucumber, 33 from watermelon, two from cantaloupe, four from cucuzzi gourd, two from honeydew, 10 from cocklebur, and three from unknown cucurbit hosts) and three isolates of C. magna (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 Collection. Twenty-eight isolates, selected to represent all vegetative compatibility groups (VCGs) and several previously identified races, were 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 composed of 28 cucumber and two cantaloupe isolates, and VCG 2 was composed of 33 watermelon, two cucuzzi gourd isolates, and a single isolate with an unknown host origin. Seven older cucumber isolates, collected prior to 1986, belonged to a third VCG (VCG 3). Of the three VCGs pathogenic on the cucurbit differentials, two virulence phenotypes could be identified. Isolates in VCG 1 gave disease reactions similar to isolates previously described as race 1, whereas isolates in VCG 2 gave disease 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.