Cotoneaster is an ornamental shrub valued for showy flowers, berries and architecture as well as the ability tolerate adverse conditions under which other taxa fail. Cotoneaster is a highly diverse genus of over 400 species, of which few are available in the US nursery trade. Some species commercially available have been identified as potentially invasive in the state of Oregon and also are susceptible to the bacterial disease fire blight. Cotoneaster selections with reduced fertility and disease resistance would be desirable characteristics for low input landscape plants. A goal of my research was to characterize Cotoneaster spp. to provide information for the rationale planning and development of novel clones to meet these horticultural goals. In the first study, genome sizes were estimated using flow cytometery and ploidy levels were inferred using holoploid genome size. Observed differences in monoploid genome sizes translate to a difference in chromosome size. Differences in chromosome size may present a reproductive barrier when they are large. This may pose a challenge in wide crosses, but may be utilized to achieve sterility in the F1 interspecific hybrid population. Differences in genome size are not related to taxonomic ordering, so wide inter sectional and inter subgeneric crosses may be possible. In the second study, susceptibility of Cotoneaster to fire blight was measured on plants inoculated with Erwinia amylovora strain Ea153. In greenhouse assays conducted over two years, plants were inoculated by cutting leaves with scissors infested with the pathogen. Some species were 'highly susceptible' to fire blight where plants were killed to the ground, and others were rated 'highly resistant' and no lesions were observed. Seventeen accessions were rated as resistant to fire blight This research provides the first report of ploidy, genome sizes, and susceptibility of species of Cotoneaster to fire blight. Collectively this research provides a toolbox for a breeder to tackle the challenge of creating disease resistant cultivars with reduced fertility. / Graduation date: 2013