Key message: Rdr3 is a novel resistance gene of black spot in roses that maps to a chromosome 6 homolog. A new DNA test was developed and can be used to pyramid black spot resistance in roses. Abstract: Diplocarpon rosae, the cause of rose black spot, is one of the most devastating foliar pathogens of cultivated roses (Rosa spp.). The primary method of disease control is fungicides, and they are viewed unfavorably by home gardeners due to potential environmental and health impacts. Planting rose cultivars with genetic resistance to black spot can reduce many of the fungicide applications needed in an integrated pest management system. To date, four resistance genes have been identified in roses (Rdr1, Rdr2, Rdr3, and Rdr4). Rdr3 was never mapped and is thought to be unique from Rdr1 and Rdr2. It is unknown whether it is an allele of Rdr4. To assess the novelty of Rdr3, a mapping population was created by crossing the Rdr3 containing cultivar George Vancouver with the susceptible cultivar Morden Blush. The mapping population was genotyped with the WagRhSNP 68 K Axiom array and mapped using the ‘polymapR’ package. Rdr3 was mapped to a chromosome 6 homolog confirming it is different from Rdr1 and Rdr2, found on chromosome 1, and from Rdr4, found on chromosome 5. The mapping information was used in conjunction with the Rosa chinensis genome assembly to develop new tightly linked SSRs for marker-assisted breeding. Three markers were able to predict the presence of Rdr3 in a 63-cultivar validation set. Additionally, 12 cultivars appear to have resistance genes other than Rdr3. The improved diagnostic markers will be a great asset to the rose-breeding community toward developing new black spot-resistant cultivars.
Bibliographical noteFunding Information:
This research was funded through the USDA’s National Institute of Food and Agriculture—Specialty Crop Research Initiative project, ‘RosBREED: Combining Disease Resistance and Horticultural Quality in New Rosaceous Cultivars’ (2014-51181-22378).
The authors would like to thank April Nyberg, Jaimie Green, Katie Pardee, Andy Sherwood, Sarah Kummeth, Drew Zagala, and Seth Heder for their technical support and Bailey Nurseries and Star ? Roses and Plants for donating rose plants. The authors also would like to thank Mandie Driskill for technical support in testing the diagnostic markers.