The development of strategies for the breeding of black spot resistant roses

The identification of roses with resistance to black spot disease and doubling the genome of diploid roses to facilitate hybridization with tetraploid cultivars are both important in rose breeding and were investigated in this thesis.

Thirty five rose species and cultivars were challenged with four pathotypes of Diplocarpon rosae, the causal agent of black spot disease in roses.
Patterns of resistance and susceptibility in leaf disc assays identified novel sources of genetic resistance and indicated crosses that might give insight into the inheritance of black spot disease resistance. Only roses from the section Cinnamomeae, or with Cinnamomeae ancestry, were susceptible to pathotype DA2, suggesting a relationship between resistance to black spot disease and the phylogeny of the sub-genus Eurosa. The possibility that the pathosystem involving pathotype DA2 differs from that of the other three pathotypes, DAl, ZM1 and CW1, is discussed. Hybrids between a resistant and a susceptible
variety of Rosa rugosa proved susceptible, indicating that the resistance was not inherited as a single dominant gene in this case.

Chromosome doubling was induced in vitro in a R. rugosa hybrid using oryzalin as the spindle inhibitor. Nodal sections, 2 mm long, were exposed to 2.5 μM or 5 μM oryzalin and 10 mm nodal sections were exposed to 5 uM oryzalin for 0 (controls), 6, 12, 24 and 48 h. The ploidy of the emergent shoots was determined by flow cytometry. The frequency of tetraploid leaves that developed from 2 mm nodal sections exposed to 5 uM oryzalin peaked at 12 h exposure (35 % tetraploidy) but fell after longer exposures. Fewer tetraploid leaves were found when 2 mm nodes were exposed to 2.5 μM oryzalin for 6 and 12 h, indicating that it took longer for a spindle inhibiting concentration of oryzalin to build up in the meristem. However, the frequencies of tetraploid leaves rose with exposure time and at 48 h, 44 % were tetraploid. In treatments with 5 jiM oryzalin, the frequencies of tetraploid leaves were lower, at equivalent exposure times, in 10 mm nodes than 2 mm nodes. This suggests that oryzalin diffused to the meristem via the cut surfaces and that access via the epidermis and cuticle was impeded.

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