Breeding Disease-Resistant Roses
There are many thousands of commercial rose cultivars in circulation, most of which are complex tetraploid or triploid hybrids derived from eight to ten wild diploid and a few tetraploid rose species. Rose breeding has predominantly focused on the development of cultivars with the desired combinations of color, form, fragrance, and cold hardiness.
The greatest dangers for cultivated roses in the world are disease pathogens and insect infestations. Pathogens and pests in commercial roses are generally controlled with pesticides, often at substantial expense to the grower. As a result, the incorporation of disease resistance, especially to the black spot fungus (Diplocarpon rosae. [Lib.] Wolf), powdery mildew (Sphaerotheca pannosa Wallr. [ex Fr.] Lev), rust (Phragmidium spp.), cercospora leaf spot (Cercospora spp.), and rose rosette disease caused by the Rose Rosette Virus into heterozygous rose genotypes is greatly desired. Within the USA, the most damaging diseases on roses are black spot and rose rosette disease.
Rose growers produced ~37 million garden roses worth $203 million in 2014 but only ~25 million bushes worth $168 million in 2019. (Census of Horticulture Specialties, 2014; 2019). These losses are, in large part, due to the damage caused by black spot and the Rose Rosette Disease (RRD). RRD is difficult to control because all major rose cultivars are susceptible, its vector is a microscopic wind-dispersed eriophyid mite, and it is spread by the nationwide distribution of asymptomatic infected rose plants to wholesale and retail markets. Extreme outbreaks have prompted nurseries, garden centers, and landscape businesses to discontinue or reduce the number of roses in their inventories or close their business resulting in the loss of thousands of jobs. Similarly, most major roses are susceptible to black spot which is controlled by repeated fungicide applications. Due to increasing aversion by the public to the use of pesticides in landscapes, today’s most successful garden roses have good black spot resistance. This emphasizes the need to strategically incorporate durable black spot resistance into new RRD-resistant roses.
For the last 200 years, rose breeders have made tremendous advances, and a rainbow of colors and a wide spectrum of flower and plant forms are available. We owe much to those who created this cornucopia of beauty. However, extensive research remains to be done. Too little effort has been devoted to developing sustainable rose cultivars – those well adapted to the abiotic (heat, cold, salt, drought) and biotic (disease and pests) stresses in our gardens. And this is what consumers want – beautiful well-adapted roses that need a minimum of maintenance.
The Rose Breeding and Genetics Program uses a multidisciplinary team to combine the traditional sciences of breeding, genetics, plant pathology, entomology, and plant physiology with the tools of biotechnology. The main objective is to develop unique sources of disease-resistant germplasm and the methodology to rapidly incorporate these adaptation traits into our commercial roses. We do this by taking a coordinated transdisciplinary approach that involves collaborative research, demonstration trials, and close cooperation with the industry and rose community.