Cotoneaster (Rosaceae) is a large genus of deciduous and evergreen plants that range from prostrate groundcovers to small trees. The genus is separated into two subgenera including Chaenopetalum with white flowers and Cotoneaster with pink flowers. The former includes common cultivars such as Coral Beauty. Cotoneasters are generally utilitarian shrubs used in mass, often as hedge plants. There has been relatively little modern breeding, although there are many available cultivars. Dirr (2009) noted that beyond Dr. John Ruter at University of Georgia, virtually no breeders in the United States are pursuing efforts to improve and introduce cultivars of this utilitarian genus. Primary challenges for breeders to address are weediness of some species (e.g., C. lacteus), poor growth habit, and susceptibility to fire blight caused by Erwinia amylovora (Contreras 2018). Other groups have documented resistance to fire blight among cotoneasters (Davis and Peterson 1976; Lecomte and Cadic 1993; Losing 1992) or performed some breeding and selection (Bellenot-Kapusta et al. 2002; Persiel and Zeller 1981), but fewer studies have been performed in recent decades.
Origin
‘Emerald Beauty’ USPP32,308 and ‘Emerald Sprite’ USPP31,719 originated as seeds collected from Cotoneaster ×suecicus ‘Coral Beauty’ during 2011. ‘Coral Beauty’ is a diploid (2n = 2x = 34; 2C = 1.53 pg) from which the seedlings were derived after flowers were hand-pollinated using pollen from C. divarcatus (2n = 4x = 68; 2C = 3.05 pg). Flow cytometry confirmed that the seedlings were not triploid; therefore, they likely resulted from accidental self-pollination (Rothleutner et al. 2016) (Table 1). Other seedlings from this and other interploidy crosses had intermediate genome sizes ranging from 2.35 to 2.43 pg (Table 1), and they had morphology shared with the pollen parent (e.g., pink flowers and leaf morphology). ‘Emerald Beauty’ and ‘Emerald Sprite’ were originally accessioned and evaluated as H2011-02-005 and H2011-02-001, respectively. Both cultivars were grown in containers during 2012 for observation and selected for propagation in 2013. They were propagated by stem cuttings using a 2:1 mix comprising a peat-based mix (Sunshine® #1; Sun Gro Horticulture, Agawam, MA, USA) and perlite set, respectively, in community propagation flats (Deep Propagation flat 2401B; Anderson Pots, Portland, OR, USA). Cuttings rooted readily after treatment with 1000 ppm IBA quick dip under intermittent mist. Clones produced from serial asexual propagation demonstrated the stability of its traits from 2013 through 2018 (‘Emerald Sprite’) or 2019 (‘Emerald Beauty’); at those times, each was released from the Oregon Agricultural Experiment Station and United States plant patents were submitted and subsequently granted the USPP numbers listed. Both selections were vouchered and submitted to the Oregon State University Herbarium, where ‘Emerald Beauty’ was accessioned as OSC-V-256544 (electronic record available at https://oregonflora.org/collections/individual/index.php?occid=4191186) and ‘Emerald Sprite was accessioned as OSC-V-256545 (electronic record available at https://oregonflora.org/imglib/OSU_V/OSC-V-256/OSC-V-256545.JPG).
Flow cytometry analysis of Cotoneaster parents and progeny used to calculate holoploid DNA content (2C) and infer ploidy level according to the methods of Rothleutner et al. (2016).
Description
‘Emerald Sprite’ is a compact cultivar with a cushion-like, mounding habit, extremely dense foliage, and short internodes (Figs. 1, 3). The maximum height is ∼31 cm; however, more commonly, plants will have a height of 20 cm and width of 60 cm. It is evergreen to semi-evergreen, depending on climate, with alternately arranged leaves that have a length of 1.7 cm and width of 0.8 cm. Young leaves are 183A (greyed-purple group) and mature to N134A adaxially and 138B abaxially (Royal Horticultural Society 2015).
‘Emerald Sprite’ (A) in a landscape planting at 2 years old from cutting, photographed on 26 May 2017, and (B) in a 26.5-L container at approximately 4 years old from cutting, photographed on 21 Jun 2018. Both photos are of plants growing in Corvallis, OR, USA.
Citation: HortScience 58, 10; 10.21273/HORTSCI17329-23
Flowers are solitary or in groups of two to five, and they are produced during May to June in Corvallis, OR, USA. Flower buds are 68A (red purple group) to N57A and open, with five petals that are NN155C (white group). Fruit is not prolific and is generally obovate, with a diameter of 0.5 cm and length of 0.5 cm. The base color is greyed-red group 180C, with a somewhat irregular coloration, and is generally lighter than that of other cultivars with green undertones.
‘Emerald Beauty’ is a semi-rounded, compact shrub with very regular branching that requires little pruning in production (Figs. 2, 3) or in landscapes. At maturity, it is 50 to 60 cm tall and 90 to 100 cm wide. It is evergreen to semi-evergreen, depending on climate, with alternately arranged leaves that are ∼2 cm long and 1 cm wide. During the growing season, foliage is green group N139A adaxially and greyed-green group 191B abaxially. Its fall color is highly variable, with predominating color 184A greyed-purple group.
Finished 2-year-old plants of ‘Emerald Beauty’ being grown in 11.4-L containers that were pruned only twice during production. Photo taken on 25 Jul 2018, in Corvallis, OR, USA.
Citation: HortScience 58, 10; 10.21273/HORTSCI17329-23
Unpruned containers of cotoneasters from left to right: ‘Coral Beauty’ (37.9-L container), ‘Emerald Sprite’ (26.5-L container), and ‘Emerald Beauty’ (26.5-L container). Photographed on 19 May 2019, in Corvallis, OR, USA.
Citation: HortScience 58, 10; 10.21273/HORTSCI17329-23
Flowers are solitary or in groups of two to five and produced during May to June in Corvallis, OR, USA. Flowers have five petals that are white group NN155C and are 1 cm in diameter. Fruit are obovate, with a diameter of 0.8 cm and length of 0.85 cm. The sun-exposed base color is red group 42B, with a somewhat irregular blush coloration where shaded that is red group 39B.
Replicates of ‘Emerald Sprite’ and ‘Emerald Beauty’ produced from stem cuttings were included in a glasshouse study to evaluate fire blight resistance of hybrids and parents during 2014 (Rothleutner et al. 2014) (Table 2). Plants were inoculated on 28 Apr 2014, with a virulent strain (Ea153) of Erwinia amylovora by bisecting the two youngest leaves on vigorously growing shoots, according to Rothleutner et al. (2014). ‘Emerald Sprite’ was symptom-free during this study (0% shoot infection), and ‘Emerald Beauty’ exhibited minor disease symptoms (1% shoot infection). ‘Coral Beauty’ had 11.1% mean shoot infection during that evaluation. We have not inoculated these cultivars with all isolates under all conditions; however, during another study (Table 2), ‘Emerald Beauty’, ‘Emerald Sprite’, and ‘Coral Beauty’ were inoculated with the highly virulent but extremely rare isolate LA635 that has a natural mutation in avrRpt2 that encodes for a type III secretion effector. During 2018 and 2019, these cultivars were evaluated in a series of experiments in growth chambers and glasshouse inoculations (Neill et al. 2021). Over five experiments, ‘Emerald Sprite’ and ‘Emerald Beauty’ tended to have improved resistance to both isolates, although this was not reflected in every experiment (Table 2). Of note was the major difference between infection of ‘Emerald Sprite’ (0%) and ‘Emerald Beauty’ (3%) with ‘Coral Beauty’ (58%) when inoculated in a glasshouse with a common virulent strain (Ea153). This experiment is more applicable to production or end use, during which plants experience higher temperatures and potentially periodic water stress as compared with the growth chamber experiments. ‘Emerald Sprite’ exhibited lower mean shoot infection than ‘Coral Beauty’ in all experiments, although it was not statistically different in several experiments. ‘Emerald Beauty’ had lower shoot infection than ‘Coral Beauty’, except for the 2018 inoculation with Ea153 and the 2019 inoculation with LA635.
Percent shoot necrosis of Cotoneaster ×suecicus ‘Emerald Sprite’ PP31719, C. ×suecicus ‘Emerald Beauty’ PP32308, and C. ×suecicus ‘Coral Beauty’ inoculated in growth chambers and a glasshouse with a wild type (strain Ea153) or in growth chambers with an avrRpt2 mutant of Erwinia amylovora (strain LA635). Both strains at a concentration of 109 cfu⋅mL−1, with a foliar bisection assay. Lesion lengths were measured 8 weeks after inoculation. Data from Neill et al. (2021).
Culture and Use
Both ‘Emerald Beauty’ and ‘Emerald Sprite’ exhibit cultural requirements similar to ‘Coral Beauty’, although they have not been observed extensively outside of Corvallis, OR, USA, in common garden trials. Briefly, cotoneasters such as Coral Beauty and these new cultivars perform best in well-drained soils and can tolerate neglect. ‘Emerald Beauty’ performs similarly to ‘Coral Beauty’ in landscapes, fills the same landscape role, and may be used for massing in commercial or residential landscapes; however, it requires less pruning to maintain uniform hedges. ‘Emerald Sprite’ is extremely compact and has been observed to be less durable in low-input landscapes; therefore, it is recommended for residential landscapes as foundation or edging.
Availability
Both cultivars are available for license from PlantHaven International (http://planthaven.com). Unrooted cuttings are available from Oregon State University to support stock increase. Contact the author for information.
References Cited
Bellenot-Kapusta V, Chartier R, Brisset MN, Paulin JP. 2002. Selection of a genotype of Cotoneaster with a high level of resistance to fire blight. Acta Hortic. 590:385–392. https://doi.org/10.17660/ActaHortic.2002.590.59.
Contreras RN. 2018. Breeding for form and fire blight resistance in Cotoneaster. Acta Hortic. 1191:83–87. https://doi.org/10.17660/ActaHortic.2018.1191.12.
Davis SH Jr , Peterson LJ. 1976. Susceptibility of Cotoneaster to fire blight. J Arboric. 2:90–91. https://doi.org/10.48044/jauf.1976.021.
Dirr MA. 2009. Manual of woody landscape plants: Their identification, ornamental characteristics, culture, propagation and uses (6th ed). Stipes Publishing, Champaign, IL, USA.
Lecomte P, Cadic A. 1993. Further results on shoot susceptibility of Cotoneaster to fire blight. Acta Hortic. 338:407–412. https://doi.org/10.17660/ActaHortic.1993.338.67.
Losing H. 1992. Cotoneaster dammeri: Fire blight (Erwinia amylovora) resistant cultivars in Germany. Proc Int Plant Propagators’ Soc. 42:123–124.
Neill KE, Contreras RN, Stockwell VO, Chen H. 2021. Screening Cotoneaster sp. for resistance to fire blight using foliar inoculation with two strains of Erwinia amylovora. HortScience. 56:824–830. https://doi.org/10.21273/HORTSCI15872-21.
Persiel F, Zeller W. 1981. Some progress in breeding Cotoneaster for resistance to fire blight, Erwinia amylovora (Burr.) Winslow et al. Acta Hortic. 117:83–88. https://doi.org/10.17660/ActaHortic.1981.117.13.
Rothleutner JJ, Contreras RN, Stockwell VO, Owen JS. 2014. Screening Cotoneaster for resistance to fire blight by artificial inoculation. HortScience. 49:1480–1485. https://doi.org/10.21273/HORTSCI.49.12.1480.
Rothleutner JJ, Friddle MW, Contreras RN. 2016. Ploidy levels, relative genome sizes, and base pair composition in Cotoneaster. J Am Soc Hortic Sci. 141:457–466. https://doi.org/10.21273/JASHS03776-16.
Royal Horticultural Society. 2015. Royal Horticultural Society Colour Chart (6th ed). Royal Horticultural Society, London, UK.
