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ISHS Acta Horticulturae 1191: III International Symposium on Woody Ornamentals of the Temperate Zone

Breeding for form and fire blight resistance in Cotoneaster

Author:   R.N. Contreras
Keywords:   Erwinia amylovora, disease resistance, interploid crossing, triploid
DOI:   10.17660/ActaHortic.2018.1191.12
Abstract:
At one time, cotoneasters were nearly ubiquitous in nursery production for use as tough landscape shrubs that offered low input options and provided multiseason interest. However, over the past decade or more, production has dwindled due to fire blight pressure and a lack of new introductions. Perhaps this is understandable, as modern consumers demand year-round color but as resources, particularly water, grow scarcer there will be increased demand for new cultivars of low-input, drought tolerant shrubs. In addition to being tough, Cotoneaster is rich with diversity of many types that could be exploited by breeders - growth habit, leaf morphology, flower form, fruit, cytotype, and disease resistance. Controlled crosses were designed to develop improved cotoneasters based on data from prior work to assess disease resistance using glasshouse inoculation as well as ploidy analysis. All crosses used 'Coral Beauty', a diploid, as the pistillate parent because it produces sexually derived progeny, as opposed to the prevalence of apomixis among polyploids. We made intra- and interploid crosses within and between subgenera as well as crossing the relatively susceptible diploid 'Coral Beauty' (mean shoot necrosis = 11%) with both susceptible and resistant pollen parents. There was no triploid block in our interploid crosses and we recovered triploid progeny using the tetraploid parents Cotoneaster divaricatus Rehder & E.H. Wilson, C. simonsii Baker, and C. splendens Flinck & B. Hylmö as pollen parents. Susceptibility to fire blight was unpredictable in our limited number of progeny, which showed transgressive segregation for fire blight resistance, ranging from 0 to 29% shoot necrosis. A triploid hybrid resulting from an interploidy and intersubgeneric cross between 'Coral Beauty' × C. splendens showed that pink, cup-like flowers of subgenus Cotoneaster are dominant to the white, fully opening flowers characteristic of subgenus Chaenopetalum. Three promising selections from our early work have shown 0% shoot necrosis, compact growth habit, high success of rooting, and very fast container production.

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