Cotoneaster is a diverse genus of over 400 species ranging in form from groundcovers to trees. The center of diversity for the genus is in Tibet and the Himalayas, but species are native across continental Asia, North Africa, and Europe (Bartish et al., 2001; Dickoré and Kasperek, 2010; Fryer and Hylmö, 2009). Cotoneaster is commonly used in managed landscapes as durable, low-maintenance ornamental shrubs where they are valued for multiseason interest for flowers, foliage, and fruiting characteristics. Although Cotoneaster generally is robust and easy to cultivate, many taxa are susceptible to the bacterial disease fire blight caused by Erwinia amylovora. Disease susceptibility limits the potential of the genus in landscape applications. For example, in some areas of Europe, cotoneasters have been banned as landscape ornamentals as a result of susceptibility to fire blight and concern that diseased Cotoneaster plants in the landscape may provide an inoculum source for disease in pear and apple orchards (B. Duffy, personal communication).
Erwinia amylovora is native to the United States and has a host range limited within members of Rosaceae, most commonly affecting members of the apple subfamily (Maloideae) (van der Zwet and Beer, 1999). The disease can especially be problematic in areas where warm temperatures, rain, and humid conditions favor bacterial growth and disease development such as the south, east, and midwest regions of the United States as well as areas in Europe where the pathogen was introduced. The pathogen overwinters in woody host tissues where it replicates through the spring and summer producing a bacterial exudate that provides a source of inoculum for infection within and among nearby plants (van der Zwet and Beer, 1999). The bacteria are dispersed primarily through rain, wind, and visiting insects. The pathogen enters the plant through wounds or natural openings; flowers are particularly vulnerable. Infected shoots first appear water-soaked and then develop a scorched appearance, often with a characteristic shepherd’s crook. When bark of infected plants is removed, discolored vascular tissue may be seen. Fire blight will quickly kill entire plants if the disease progresses to the crown. In addition to threatening the health of the plant, the necrotic tissue is unsightly and devalues nursery and landscape plants. Fire blight is managed through labor-intensive sanitation or prophylactic applications of copper or antibiotics, where permitted (van der Zwet and Beer, 1999). Host resistance, as a first line of defense in an integrated pest management plan, could greatly reduce the need for chemical inputs and allow for more extensive planting of resistant taxa.
Screening for fire blight resistance in ornamentals such as Malus Tourn ex L., Pyrus L., Pyracantha M. Roem., and Cydonia Mill. has demonstrated a wide range of susceptibility within these genera (Bell et al., 2004; Bouma, 1990; Lespinasse and Aldwinckle, 2000; van der Zwet and Beer, 1999). Screening for fire blight resistance in Cotoneaster has been relatively limited with only a portion of the genus screened. Previous screenings of Cotoneaster germplasm has resulted in the release of cultivars with fire blight resistance including C. salicifolius Franchet ‘October Glory’ and ‘Willeki’, C. henryanus (C.K. Schneider) Rehder & E.H. Wilson ‘Corina’ (Fryer and Hylmö, 2009), and C. dammeri C.K. Schneider ‘Eicholz’ (Bellenot-Kapusta et al., 2002), ‘Holsteins Resi’, and ‘Thiensen’ (Losing, 1992). Persiel and Zeller (1978, 1981, 1990) showed there was varying resistance within sexual populations of diploid species, which has allowed for selection of resistant taxa.
Many species of Cotoneaster are reported to be apomictic clones (Dickoré and Kasperek, 2010; Fryer and Hylmö, 2009) and currently there are no known selections for fire blight resistance from these species. In addition, most reports on fire blight resistance in Cotoneaster have been observational studies under landscape conditions without testing by artificial inoculation (Davis and Peterson, 1976; Hodgkin and Fletcher, 1965; Jorgensen and Jensen, 1978; New Jersey Agricultural Experiment Station, 1932; Zeller, 1979). Disease occurrence often varies between years and environments; therefore, results from observational surveys under low disease pressure may be inconclusive. Examination of disease development in Cotoneaster under controlled artificial inoculation may clarify the list of resistant taxa. Our objective was to evaluate susceptibility of a collection of Cotoneaster after inoculation with a virulent strain of fire blight under glasshouse conditions.
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