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Doctoral thesis, 2015

Resistance against fruit tree canker in apple

Ghasemkhani, Marjan


Neonectria ditissima (formerly Neonectria galligena, anamorph Cylindrocarpon heteronema) is the causal agent of fruit tree canker, which is regarded as a serious economic problem in apple production. This fungus is closely associated with mild and wet conditions, and climate therefore has an important impact on its geographic distribution. Notable damage to apple trees is especially common in, e.g., North-Western Europe. Complete resistance has not yet been reported. In addition, no major genes have as yet been identified and resistance appears to be controlled mainly by additive gene action. Apple cultivars varied significantly in level of partial resistance towards apple canker after artificial inoculation of detached shoots and/or potted trees with macroconidia of N. ditissima. The inter-specific crabapple hybrid ‘Prairifire’ showed the least damage, indicating that valuable sources of resistance can be found in Malus species. Neonectria ditissima could be detected in artificially infected apple trees using a species-specific β-tubulin gene-based qPCR assay two months after inoculation. When quantified in different cultivars, a positive relationship between N. ditissima biomass and the size of canker-induced lesions was found. Interaction between plant and pathogen was studied using light microscopy of apple woody tissue; fungal hyphae were found in all cell types, and gel formation was observed in both a susceptible and a resistant cultivar but infection appears to progress more rapidly in the susceptible cultivar. Genetic variation among different N. ditissima isolates was investigated using SSR and AFLP markers. Comparatively low levels of genetic differentiation among orchards indicate that gene flow is prominent. Analyses of single-ascospore isolates from the same perithecia suggest that this fungus is heterothallic and therefore outcrossing. Furthermore, a focused genomics approach was applied in order to identify differentially expressed genes in response to fungal attack on apple trees. Obtained data indicate that apple cultivars inoculated with N. ditissima exhibit significant upregulation of defence-related genes and genes involved in detoxification, peroxidase-related reactions, phenylpropanoid metabolism and the lignification process.


anatomy; β-tubulin gene; defence mechanism; genetic variation; mating system; qPCR; RNA-Seq; transcriptome

Published in

Acta Universitatis Agriculturae Sueciae
2015, number: 2015:77
ISBN: 978-91-576-8352-6, eISBN: 978-91-576-8353-3
Publisher: Department of Plant Breeding, Swedish University of Agricultural Sciences

    SLU Authors

    UKÄ Subject classification

    Plant Biotechnology

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