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Eleven rose ( Rosa  spp.) cultivars and advanced lines from the Morden rose breeding program were tested in controlled conditions for five components of partial resistance to two isolates of  Diplocarpon rosae  Wolf, the causal agent of black spot. The isolates were collected from diseased rose leaves in 1995 at Morden, Man., and in Ottawa, Ont. The components examined include incubation period (IP), leaf area with symptoms (LAS), number of lesions (NL), lesion length (LL), and sporulation capacity (SC). Differences in cultivar responses were found for all components. Of the 11 rose genotypes, no visible symptoms developed on line 91V8T201 even at 14 days after inoculation. This line was considered completely resistant to the two isolates tested. X6, T5, and `Prairie Joy' had a high level of partial resistance. N3 and `Cuthbert Grant' were intermediate. The remaining five cultivars or lines exhibited a low level of partial resistance. Linear correlations among LAS, IP, NL, and LL were high. Correlations between SC with IP, NL, or LL were nonsignificant. Although each of the five components can be used in selection for partial resistance, any one component may not represent the complete potential for a particular rose genotype. Results of this study suggest that LAS and SC are preferable as criteria for resistance to minimize the labor requirement when evaluating large numbers of rose genotypes in the greenhouse.

Components of Partial Resistance to Black Spot Disease (Diplocarpon rosae Wolf) in Garden Roses