COMPONENTS OF PARTIAL RESISTANCE TO BLACK SPOT DISEASE (DIPLOCARPON ROSAE WOLF) IN GARDEN ROSES

Eleven rose (Rosa spp.) cultivars and advanced lines from the Morden r ose breeding program were tested in controlled conditions for five com ponents of partial resistance to two isolates of Diplocarpon rosae Wol f, the causal agent of black spot. The isolates were collected from di seased rose leaves in 1995 at Morden, Man., and in Ottawa, Ont. The co mponents examined include incubation period (IF), leaf area with sympt oms (LAS), number of lesions (NL), lesion length (LL), and sporulation capacity (SC). Differences in cultivar responses were found for all c omponents. Of the 11 rose genotypes, no visible symptoms developed on line 91V8T201 even at 14 days after inoculation, This line was conside red completely resistant to the two isolates tested, X6, T5, and 'Prai rie Joy' had a high level of partial resistance. N3 and 'Cuthbert Gran t' 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 se lection for partial resistance, any one component may not represent th e complete potential for a particular rose genotype. Results of this s tudy suggest that LAS and SC are preferable as criteria for resistance to minimize the labor requirement when evaluating large numbers of ro se genotypes in the greenhouse.