2018 ASHS Annual Conference

The ornamental quality of outdoor grown roses (Rosa hybrida) is under constant threat from foliar diseases, such as black spot caused by Diplocarpon rosae. Fungicides are primarily used to manage black spot; however, there is a high consumer demand for disease resistant roses which eliminate the need for chemical inputs. Phenotyping with 12 D. rosae races was conducted to better characterize resistance in four popular polyploid rose cultivars (Brite Eyes^TM, High Voltage^TM, Lemon Fizz^TM, and Morden Blush). Subsequently, two populations (‘Morden Blush’ × Brite Eyes^TM and High Voltage^TM × Lemon Fizz^TM) were developed to study resistance segregation and map the genes mediating black spot resistance using the rose Axiom array. ‘Morden Blush’ was susceptible to all races while the remaining three cultivars displayed differing disease responses. A 1:1 segregation ratio was observed for the two populations where each individual was either resistant or susceptible to all races tested to date, suggesting resistance is conferred by a single resistance gene in Brite Eyes^TM and Lemon Fizz^TM. High Voltage^TM is expected to have a different resistance gene than Brite Eyes^TM and Lemon Fizz^TM based on observed disease responses. Linkage mapping in the ‘Morden Blush’ × Brite Eyes^TM population identified a single resistance gene that mapped to a chromosome 5 homeolog (Rdr4). To date, three black spot resistance genes, Rdr1, Rdr2, and Rdr3, have been identified. Rdr1 and Rdr2 both map to chromosome 1 indicating they are not allelic to Rdr4 and the location of Rdr3 is currently unknown. D. rosae races 3 and 9 are virulent on Rdr3 but avirulent on Rdr4. As such, we cannot confirm if Rdr4 is a unique gene or an allele of Rdr3. Future work will focus on developing tools for marker assisted breeding and pyramiding the identified resistance genes into new cultivars.