Pathogenicity Grouping of Isolates of Leptosphaeria maculans on Brassica napus Cultivars and Their Disease Reaction Profiles on Rapid-Cycling Brassicas. Alemu Mengistu, Assistant Researcher, Department of Plant Pathology, University of Wisconsin-Madison, Madison 53706. S. R. Rimmer, E. Koch, and P. H. Williams. Associate Professor, Department of Plant Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2; Former Postdoctoral Research Associate, and Professor, Department of Plant Pathology, University of Wisconsin-Madison, Madison 53706. Plant Dis. 75:1279-1282. Accepted for publication 7 June 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-1279.

The pathogenic variability among isolates of Leptosphaeria maculans from different geographic regions was tested using 39 isolates from North America, Europe, and Australia against Brassica napus cultivars Westar, Quinta, and Glacier. An additional 62 single-ascospore isolates from oilseed rape debris from Saskatchewan and Manitoba, Canada, and from Western Australia and New South Wales, Australia, were also tested to study pathogen variability within locations. Based on the reaction on cotyledons of Westar, Quinta, and Glacier, isolates were categorized into four pathogenicity groups (PG). PG1 isolates were distinguished by the lack of virulence on Westar, Glacier, and Quinta. PG2 isolates were virulent only on Westar but tended to give slightly more susceptible interaction phenotypes on Quinta than on Glacier. PG3 isolates were virulent on both Westar and Glacier and intermediate on Quinta. PG4 isolates were virulent on all three cultivars. All single-ascospore isolates from Canada were classified as PG2, whereas isolates from Australia were more varied in virulence on the differentials and could be classed in groups PG2, PG3, or PG4. When rapid-cycling populations of B. rapa, B. nigra, B. oleracea, B. juncea, B. napus, B. carinata, and Raphanus sativus were tested with isolates characteristic of PG1, PG2, PG3, and PG4, further discrimination among PG groups was not observed. The virulence profiles within each of the rapid-cycling cruciferous species indicated plant-to-plant variation for interaction phenotype within populations of B. rapa, B. oleracea, B. napus, and R. sativus. Through selection within the rapid-cycling base populations of these species, it may be possible to obtain inbreds with more precisely defined interaction phenotypes to specific pathogenicity groups of L. maculans.