Assessing fire blight resistance of new Cotoneaster genotypes inoculated with Erwinia amylovora in the field

Authors: GIFFEI, B - University Of California; CONTRERAS, R - Oregon State University; Stockwell, Virginia; SISNEROZ, J - University Of California; REID, S - University Of California - Cooperative Extension Service; OKI, L - University Of California

Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2019
Publication Date: N/A
Citation: N/A

Interpretive Summary: Cotoneasters are a diverse group of beautiful flowering ornamental shrubs that vary in size, shape, leaf size, and many have attractive red berries during the dreary winter months. Aside from aesthetic attributes, cotoneasters are low-maintenance plants with remarkable heat- and drought-tolerance, which makes them perfect for plantings in parking lots and along city sidewalks. Unfortunately, cotoneasters are very susceptible to a bacterial disease called fire blight caused by Erwinia amylovora. Fire blight can kill cotoneaster plants within a few weeks after infection. Consequently, landscapers avoid using these otherwise hardy and beautiful plants. The Oregon State University Ornamental Breeding program, in collaboration with USDA ARS, is working on breeding and selecting cotoneasters with resistance to fire blight, while retaining aesthetically-pleasing attributes. A selection of the new cotoneasters were tested for fire blight resistance in large field trials in California. Of those tested, two accessions were completely resistant to fire blight and did not show any symptoms of the disease. These new varieties of cotoneaster will give homeowners and city landscapers new options to grow hardy, drought-tolerant flowering shrubs to beautify urban environments, even in areas where fire blight is a constant threat.

Technical Abstract: The genus Cotoneaster Medik. encompasses hundreds of woody shrub species whose growth habits range from prostrate to upright. Cotoneaster species are desirable in landscapes for their low maintenance requirements and year-round aesthetic interest. However, the health and aesthetic quality of many Cotoneaster species is threatened by the bacterial disease “fire blight,” caused by Erwinia amylovora (Burrill 1882) Winslow et al. 1920. This study assessed fire blight resistance of three new Cotoneaster genotypes (H2011-02-001, H2011-01-002, and H2011-02-005) bred for disease resistance and two commercially available cultivars, Cotoneaster dammeri ‘Coral Beauty’ and C. dammeri ‘Lowfast’ under field conditions. Fourteen replicates of each taxa were planted in Davis, CA (USDA climate zone 9; Yolo silty clay loam soil) in May 2017. For plant establishment, irrigation at 80% of reference evapotranspiration (ETo) was provided during the first summer after planting. In the second summer after planting (2018), plants received deficit irrigation at 50% of ETo. In June 2018, seven replicates of each taxa were artificially inoculated with a local isolate of E. amylovora by bisecting the two youngest leaves on one branch with scissors dipped in a bacterial suspension of 109 CFU/ml. An additional seven replicates were artificially inoculated using the same method with sterile deionized water as a control. Fire blight susceptibility was assessed by calculating the Percent Shoot Necrosis (PSN = 100*(lesion length/total branch length)) once a week for eight weeks after inoculation. Taxa with a PSN greater than 0% were considered susceptible and taxa with a PSN equal to 0% were considered resistant. H2011-02-005 (mean PSN=14.5+/-8.4%), C. dammeri ‘Coral Beauty’ (mean PSN= 4.4+/-4.4%), and C. dammeri ‘Lowfast’ (mean PSN=12.3+/-11.1%) showed varying levels of susceptibility. H2011-02-001 and H2011-01-002 showed no necrosis (mean PSN=0.0+/-0.0%) and were considered resistant. The two resistant genotypes are candidates for new ornamental introductions where fire blight is prevalent.