Characterization of the Rose Black Spot Pathogen and the Hunt for Resistance Genes at the University of Minnesota

Stan C. Hokanson^a*, Vance M. Whitaker^b, James Bradeen^c

^a,b Associate Professor and Graduate Student, University of Minnesota, Department of Horticultural Science, St. Paul, MN 55108 USA

^c Assistant Professor, University of Minnesota, Department of Plant Pathology, St. Paul, MN 55108 USA

Since 1990 the Woody Landscape Plant Breeding and Genetics program at the University of Minnesota has been engaged in the process of breeding cold-hardy, repeat-blooming, disease tolerant shrub roses for the garden and landscape. Early in the process it became apparent that producing cultivars with durable resistance to rose black spot (Diplocarpon rosae Wolf) would require a comprehensive, parallel, lab-based approach. Our initial effort involved a characterization of the black spot pathogen in eastern North America. Results from a physiological race reaction evaluation revealed the presence of three races of the pathogen. In addition, evaluations of several components of partial resistance in cultivated rose germplasm suggested the existence of race specific partial resistance in rose. An amplified fragment length polymorphism (AFLP) based evaluation of fifty black spot isolates collected from throughout eastern North America revealed considerable marker diversity. However, cladistic and phenetic analysis revealed no significant clustering based on host of origin, race, or geographic origin. These results suggest spread of D. rosae may be related to the vegetative mode of propagation and long distance distribution of plants in commerce. Current efforts involve a genetic analysis of major and race-specific partial resistance genes in cultivated rose germplasm. Efforts are underway to develop markers linked to major resistance genes utilizing bulk segregant analysis with markers generated with the nucleotide binding site (NBS) profiling technique. Populations have also been developed to determine the combining ability for the race specific partial resistance noted in previous studies. Finally, we have initiated a collaborative effort to simultaneously characterize black spot races from collections in Germany, England, and Minnesota. The races will be evaluated on a differential array composed of hosts utilized in the previous race differentiation evaluations. Results should allow for a more unified approach to the study of D. rosae resistance around the world.