Evaluation of Landscape Roses From the Earth-KindŽ Trials: Race-Specific Black Spot (Diplocarpon rosae Wolf) Resistance and Ploidy

The Earth-Kind® program, created at Texas A&M, serves to unify research and extension/education efforts that support sound environmental stewardship in the ornamental landscape. Regional, replicated cultivar trials of selected landscape roses are an ongoing component of Earth-Kind® research. These trials identify and promote the most adapted cultivars and are conducted without fertilizers or pesticides. Black spot (caused by Diplocarpon rosae Wolf) is the most serious disease of outdoor-grown roses worldwide due to the potential for rapid leaf yellowing and defoliation. The 17 Earth-Kind designated cultivars for the Southern US region, 30 cultivars in the Earth-Kind Brigade (Mid-US), and 20 cultivars in the Northern Earth-Kind Rose Trials (58 cultivars total; some are represented across groups) were challenged with three races of D. rosae previously characterized (races A, B, and C) at the University of Minnesota from isolates collected across Eastern North America. Detached leaf assays were used in humid chambers on newly expanded leaves. Lesion size was measured for susceptible reactions and ploidy was determined using root tip squashes. Diploid (n=14), triploid (n=21), and tetraploid (n=23) cultivars were identified. Race specific resistance was found in two of the 17, 14 of the 30, and 13 of the 20 cultivar groups, respectively. To our knowledge, this is the first widespread screen of rose cultivars using characterized races of D. rosae.  Cultivars resistant to all three races include: ‘BAIine’ (Yellow Submarine^TM), ‘Radbrite’ (Brite Eyes^TM), ‘Radcon’ (Pink Knock Out®), and ‘Radrazz’ (Knock Out®). ‘Radyod’ (Blushing Knock Out®), interestingly, is a sport of ‘Radrazz’ and is susceptible to race A. Generalized trends for race specific resistance were found among germplasm groups based on breeding program; e.g. many Buck roses were resistant to race B and many Explorer® roses were susceptible to race C. Variation in lesion size for susceptible reactions was found.  The application of this data includes: 1) comparing lesion size data from this study with a growing body of field resistance data from Earth-Kind field trials where inoculum is not controlled or quantified, 2) employ detached leaf assays as a method for pre-screening roses prior to inclusion in Earth-Kind field trials, 3). better understand races present in rose gardens based upon infection patterns on plantings of cultivars from this study differing for race specific resistance, and 4). provide breeders with important resistance and ploidy data which can be used to effectively pyramid race specific resistances and develop additional resistant cultivars.