VIEW ARTICLE

Resistance

Variation in Virulence in Successive Single-Zoospore Propagations of Phytophthora megasperma f. sp. glycinea. F. S. Rutherford, Research associate, Research Centre, Agriculture Canada, University Sub Post Office, London, Ontario, N6A 5B7; E. W. B. Ward(2), and R. I. Buzzell(3). (2)Principal plant pathologist, Research Centre, Agriculture Canada, University Sub Post Office, London, Ontario, N6A 5B7; (3)Plant geneticist, Research Station, Agriculture Canada, Harrow, Ontario, N0R 1G0. Phytopathology 75:371-374. Accepted for publication 2 October 1984. Copyright 1985 Department of Agriculture, Government of Canada. DOI: 10.1094/Phyto-75-371.

Variation in virulence was studied in successive single-zoospore propagations involving isolates of races 3 and 6 of Phytophthora megasperma f. sp. glycinea. Following zoospore inoculation of etiolated hypocotyls of soybean cultivars Wayne, Harosoy, and nine other cultivars each carrying a different Rps gene for resistance to P. megasperma f. sp. glycinea, the gain and loss of virulence was observed in successive single-zoospore propagations and in mass mycelial transfers in one race 3 lineage. Less variability was observed following mass mycelial transfers than following single-zoospore propagations. Isolates of races 1 and 3, lacking virulence on all 11 soybean cultivars, retained the ability to infect and elicit glyceollin production; this demonstrated that loss of virulence following single-zoospore propagation was not necessarily associated with loss of pathogenicity.