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Dissecting black spot resistance in polyploid hybrid roses

Jason Zurn: USDA-ARS National Clonal Germplasm Repository

<div>Devastating foliar diseases, such as black spot caused by <em>Diplocarpon rosae</em>, pose constant threats to the ornamental quality of outdoor grown roses. Black spot is primarily managed though the use of fungicides, however, there is a high demand for resistant roses which require low chemical inputs. To better characterize resistance in four popular polyploid rose cultivars (Brite Eyes<sup>TM</sup>, High Voltage<sup>TM</sup>, Lemon Fizz<sup>TM</sup>, and Morden Blush), phenotyping was conducted with 12 <em>D. rosae </em>races. Additionally, two populations (‘Morden Blush’ × Brite Eyes<sup>TM</sup> and High Voltage<sup>TM</sup> × Lemon Fizz<sup>TM</sup>) were developed to study the segregation of resistance and to map it with 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<sup>TM</sup> and Lemon Fizz<sup>TM</sup>. High Voltage<sup>TM</sup> is expected to have a different resistance gene than Brite Eyes<sup>TM</sup> and Lemon Fizz<sup>TM</sup> based on observed disease responses. Linkage mapping in the ‘Morden Blush’ × Brite Eyes<sup>TM</sup> population identified a single resistance gene on chromosome 5 and is different from the three previously described genes. Future work will focus on developing tools for marker assisted breeding and incorporating the broad resistances into new cultivars.</div>