Novel metrics to classify fire blight resistance of 94 apple cultivars

S. A. Kostick, J. L. Norelli, K. M. Evans

Research output: Contribution to journalArticlepeer-review

31 Scopus citations


Fire blight (Erwinia amylovora), a potentially devastating disease in apple, can cause floral, fruit and structural damage and even tree death. Most commercial apple cultivars are susceptible and the resistance/susceptibility of many modern cultivars has not been evaluated. Fire blight resistance/susceptibility is difficult to phenotype due to quantitative resistance, impacts of tree vigour and environment on susceptibility, and the erratic nature of the disease. Resistance/susceptibility levels were determined for 94 apple cultivars and important breeding parents. In 2016 and 2017, multiple actively growing shoots per tree (about three trees per cultivar) were challenged with E. amylovora Ea153n via a cut-leaf inoculation method. Proportion of current season's shoot length blighted (SLB) was calculated for each shoot. To classify cultivar responses, estimated marginal SLB means were compared to four controls, representing highly susceptible (HS) to highly resistant (HR), via Dunnett's tests. Cultivar responses ranged from HS to HR with estimated marginal SLB means of 0.001–0.995 in 2016 and 0.000–0.885 in 2017. Most cultivars demonstrated similar resistance/susceptibility levels in both years (ρ = 0.657, P < 0.0001). K-means clustering was used to classify cultivars into three resistance/susceptibility groups based on incidence, average severity (SLB), and maximum severity values (maximum SLB and age of wood infected). Sixteen cultivars were consistently moderately resistant (MR) to HR while the remainder ranged from HS to MR. An updated comparison of susceptibility of important cultivars is provided. Resistance/susceptibility information gained could be used to identify genetic loci associated with resistance/susceptibility and/or inform parental selection in apple scion breeding programmes.

Original languageEnglish (US)
Pages (from-to)985-996
Number of pages12
JournalPlant Pathology
Issue number5
StatePublished - Jun 2019
Externally publishedYes

Bibliographical note

Funding Information:
The authors declare no conflict of interest. The authors would like to thank Lisa Brutcher, Nancy Buchanan, Schaefer Buchanan, Jamie Coggins and Bonnie Schonberg of Washington State University (WSU) Tree Fruit Research and Extension Center, Wenatchee, for their assistance and/or technical expertise. Also, the authors would like to thank the WSU Columbia View Orchard farm team for maintaining the planting throughout the duration of this project. USDA is an equal opportunity provider and employer. This work was partially funded by Washington Tree Fruit Research Commission projects # CP-15-100 (A, B, & C) and CP-12-104 (A & B), USDA-NIFA award # 2016-68004-24770, USDA-NIFA-SCRI project ‘RosBREED: combining disease resistance and horticultural quality in new rosaceous cultivars’ (2014-51181-22378), and USDA-NIFA Hatch project 1014919.

Publisher Copyright:
© 2019 British Society for Plant Pathology


  • Erwinia amylovora
  • Malus pumila
  • Malus × domestica
  • shoot length blighted (SLB)


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