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- Author or Editor: Virginia O. Stockwell x
Cotoneaster Medik. is a genus of ornamental landscape plants commonly affected by fire blight. Fire blight is a disease caused by the bacterial pathogen, Erwinia amylovora (Burrill) Winslow et al., that attacks a wide range of taxa in the apple subfamily (Maloideae; Rosaceae). To assess susceptibility of species and identify potential sources of resistance, we inoculated 52 taxa of Cotoneaster with E. amylovora. Disease severity was scored by percent shoot necrosis (lesion length/total shoot length). Disease screenings were conducted over 2 years and varying levels of susceptibility were observed. Some taxa were highly susceptible to fire blight and the disease resulted in whole plant mortality (C. rhytidophyllus Rehder & E.H. Wilson, C. rugosus E. Pritzel ex Diels, and C. wardii W.W. Smith). Other taxa repeatedly exhibited moderate to high levels of disease resistance [C. arbusculus G. Klotz, C. chungtinensis (T.T. Yu) J. Fryer & B. Hylmö, C. delsianus E. Pritzel var. delsianus, C. sikangensis Flinck & B. Hylmö, C. simonsii Baker, and C. splendens Flinck & Hylmö]. Ongoing studies are being conducted to determine if taxa with high levels of resistance under artificial inoculation will exhibit high levels of resistance in the field under natural disease pressure. Identifying sources of disease resistance will be useful for breeding programs to increase tolerance of these landscape plants with desirable horticultural characteristics to fire blight.
The genus Cotoneaster is composed of ≈400 species with a wide variety of growth habits and forms. These hardy landscape shrubs used to be commonplace because of their low maintenance and landscape functionality. However, the interest in and sales of cotoneaster have decreased for a variety of reasons, with the greatest being its susceptibility to a bacterial disease fire blight caused by Erwinia amylovora. The resistances of 15 different genotypes of Cotoneaster to a wild-type strain of Erwinia amylovora (Ea153) and a strain LA635 that has a natural mutation in avrRpt2 that encodes for a type III secretion effector were tested separately by inoculating leaves. Fire blight resistance was assessed by calculating the percent shoot necrosis (PSN) [PSN = 100 × (lesion length ÷ total branch length)] at 6 to 8 weeks after inoculation. Across all experiments, Cotoneaster genotypes H2011-01-002 and C. ×suecicus ‘Emerald Sprite’ consistently had the lowest PSN values when inoculated with either strain. Cotoneaster ×suecicus ‘Emerald Beauty’ was significantly more resistant to Ea153 than to LA635, whereas C. splendens was significantly more susceptible to Ea153 than to LA635.