Screening cotoneaster sp. for resistance to fire blight using foliar inoculation with two strains of erwinia amylovora

Authors: NEILL, KRISTIN - Oregon State University; CONTRERAS, RYAN - Oregon State University; Stockwell, Virginia; HSUAN, CHEN - North Carolina State University

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/25/2021
Publication Date: 6/8/2021
Citation: Neill, K.E., Contreras, R.N., Stockwell, V.O., Hsuan, C. 2021. Screening cotoneaster sp. for resistance to fire blight using foliar inoculation with two strains of erwinia amylovora. HortScience. 56(7):824-830. https://doi.org/10.21273/HORTSCI15872-21.
DOI: https://doi.org/10.21273/HORTSCI15872-21

Interpretive Summary: Cotoneasters are hardy, low maintenance landscaping shrubs that are planted around buildings or in vegetation strips in parking lots. Cotoneasters come in different shapes from large shrubs to ground-hugging shrubs, and add year-round greenery, spring or summer flowers, and red berries to urban and suburban landscapes. Unfortunately, cotoneasters are very susceptible to the bacterial disease called fire blight. Fire blight is a devastating disease of pome fruits like pear, apple, quince, and cotoneaster. Reducing fire blight damage on cotoneaster requires weekly inspection and removal of diseased branches, a process that is too labor intensive and expensive. Our project focused on screening Cotoneaster varieties or genotypes from an Oregon State University breeding program for natural resistance to fire blight. We tested the resistance of 15 different genotypes of Cotoneaster to fire blight caused by two genetically-different strains of the pathogen, a wild type strain and a naturally-occurring variant of the pathogen with a mutation in a bacterial pathogenicity gene called avrRpt2. We included the natural avrRpt2-mutant in the screening program because these specific mutants of the pathogen have been shown to cause fire blight on wild apple trees, which are otherwise immune to fire blight. We identified Cotoneaster genotypes that are resistant to fire blight, even when challenge-inoculated with the avrRpt2-mutant; thus, the resistance of cotoneaster to fire blight may be distinct than that identified in apple. Incorporation of fire blight resistance into cotoneasters with horticulturally-attractive traits will increase the desirability, versatility, and longevity of these landscaping plants.

Technical Abstract: The genus Cotoneaster is composed of around 400 species with a wide variety of growth habits and form. 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 declined for a variety of reasons, 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 = 100*(lesion length/total branch length)) six to eight 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, while C. splendens was significantly more susceptible to Ea153 than to LA635.