Characterization of Resistance to Black Spot Disease of Rosa Spp.
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Black spot disease (BSD), caused by the fungus Diplocarpon rosae Wolf, is one of the most serious diseases of garden roses. Both complete (vertical) resistance conditioned by dominant Rdr genes and partial (horizontal) resistance (PR) conditioned by multiple genes have been described. The use of resistant rose cultivars would reduce the demand of agrochemical application. The characterization of 16 genotypes using two laboratory assays, the detached leaf assay (DLA) and the whole plant inoculation (WPI) approach, indicated that these were well correlated. Thus either method could be used to assess the resistance of the plants to the BSD. Fifteen diploid hybrid populations from 10 parents segregating for black spot partial (horizontal) resistance were assessed for black spot resistance by quantifying by the percentage of the leaf area with symptoms (LAS) and lesion length (LL) measured by the diameter of the largest lesion in detached leaf assays. Nine of these populations were also evaluated in field trials by rating the incidence of damage due to the fungal infection. The narrow sense heritability of partial resistance to black spot as measured by LAS and LL data of DLA was estimated from 0.3 to 0.4 when calculated with a genetic variance analysis and from 0.7 to 0.9 when generated from mid-parent offspring regression. In the field assessments, the second year assessments were better than the assessments done the first year due to higher and more uniform inoculum levels which minimized problems with escapes. In general there was no or just low correlations between field and DLA assessments of black spot indicating that perhaps these two assessments are measuring different aspects of resistance. The narrow and broad sense heritability estimates from the combined analysis of field assessments is 0.3 and 0.4 respectively. An examination of the assessment data from the laboratory and the field showed that some seedlings were rated as resistant using both approaches. Two microsatellite markers linked with Rdr1 locus and one SCAR marker linked to Rdr3 locus were found to be germplasm specific. The hybrid population ?Golden Gardens? x ?Homerun? that segregates for race 8 resistance was phenotyped for resistance to race 8 and genotyped for 38 SSR markers to assess if any of these SSR markers were associated with Rdr3. This resistance trait from the triploid source segregated non randomly and differentially in haploid and diploid gametes. None of the SSR markers examined were associated with Rdr3.