Mapping a Novel Black Spot Resistance Locus in the Climbing Rose Brite Eyes™ (‘RADbrite’)

Abstract

Rose black spot, caused by <i>Diplocarpon rosae</i>, is one of the most devastating foliar diseases of cultivated roses (<i>Rosa</i> spp.). The globally distributed pathogen has the potential to cause large economic losses in the outdoor cultivation of roses. Fungicides are the primary method to manage the disease, but are often viewed unfavorably by home gardeners due to potential environmental and health impacts. As such, rose cultivars with genetic resistance to black spot are highly desired. The tetraploid climbing rose Brite Eyes^TM ('RADbrite') is known for its resistance to black spot. To better characterize the resistance present in Brite Eyes^TM, phenotyping was conducted on a 94 individual F₁ population developed by crossing Brite Eyes^TM to the susceptible tetraploid rose 'Morden Blush'. Brite Eyes^TM was resistant to all <i>D. rosae</i> races evaluated except for race 12. The progeny were either resistant or susceptible to all races (2, 3, 8, 9, 10, 11, and 13) evaluated. The segregation ratio was 1:1 (χ² = 0.3830, <i>P</i> = 0.5360) suggesting resistance is conferred by a single locus. The roses were genotyped with the WagRhSNP 68K Axiom array and the 'polymapR' package was used to construct a map. A single resistance locus (<i>Rdr4</i>) was identified on the long arm of chromosome 5 homoeolog 4. Three resistance loci have been previously identified (<i>Rdr1, Rdr2</i>, and <i>Rdr3</i>). Both <i>Rdr1</i> and <i>Rdr2</i> are located on a chromosome 1 homoeolog. The chromosomal location of <i>Rdr3</i> is unknown, however, races 3 and 9 are virulent on <i>Rdr3</i>. <i>Rdr4</i> is either a novel gene or an allele of <i>Rdr3</i> as it provides resistance to races 3 and 9. Due to its broad resistance, <i>Rdr4</i> is an excellent gene to introgress into new rose cultivars.