Relationship Between Pathogenicity and Phylogeny Based on Restriction Fragment Length Polymorphism in Leptosphaeria maculans. E. Koch. Department of Agronomy, University of Wisconsin, Madison 53706. Department of Plant Pathology, University of Wisconsin, Madison 53706. K. Song(1,2), T. C. Osborn(1), and P. H. Williams(2). (1)Department of Agronomy, University of Wisconsin, Madison 53706; and (2)Department of Plant Pathology, University of Wisconsin, Madison 53706.. MPMI 4:341-349. Accepted 6 March 1991. Copyright 1991 The American Phytopathological Society.

Isolates of Leptosphaeria maculans collected in widely separated geographic regions were characterized for virulence on cultivars of Brassica napus spp. oleifera and for DNA restriction fragment length polymorphisms (RFLPs). Cotyledons of the cultivars Westar, Quinta, and Glacier were inoculated with 39 isolates, and based on disease reactions, isolates were grouped as “nonaggressive” or “aggressive.” Aggressive isolates were further divided into three pathogenicity groups based on differential reactions of the three cultivars. All of the aggressive isolates and none of the nonaggressive isolates produced phytotoxins (sirodesmins) in vitro. DNAs from 28 isolates were analyzed for RFLPs with two restriction enzymes and 42 probes of cloned nuclear DNA sequences from L. maculans, and the resulting RFLP data were used to construct a phylogenetic tree. For many of the probes, aggressive and nonaggressive isolates had different RFLP patterns and hybridization intensities, and they were separated into two phylogenetically distant groups. Aggressive isolates were subdivided into more closely related phylogenetic groups, which in some cases corresponded to their pathogenicity groups. These results suggest that aggressive and nonaggressive isolates may belong to different species, and that the phylogeny of aggressive isolates is partially related to their pathogenic performance.