Variation in Host and Pathogen in the Neonectria/Malus Interaction; toward an Understanding of the Genetic Basis of Resistance to European Canker

Abstract

Apple canker caused by the phytopathogenic fungus Neonectria ditissima is an economically important disease, which has spread in recent years to almost all pome-producing regions of the world. N. ditissima is able to cross-infect a wide range of apple varieties and causes branch and trunk lesions, known as cankers. Most modern apple varieties are susceptible and in extreme cases suffer from high mortality (up to 50%) in the early phase of orchard establishment. There is no known race structure of the pathogen and the global level of genetic diversity of the pathogen population is unknown. Resistance breeding is underway in many global breeding programmes, but nevertheless, a total resistance to canker has not yet been demonstrated. Here we present preliminary data from a survey of the phylogenetic relationships between global isolates of N. ditissima which reveals only slight evidence for population structure. In addition we report the results of four rapid screening tests to assess the response to N. ditissima in different apple scion and rootstock varieties, which reveals abundant variation in resistance responses in both cultivar and rootstock material. Further seedling tests show that the segregation patterns of resistance and susceptibility vary widely between crosses. We discuss inconsistencies in test performance with field observations and discuss future research opportunities in this area.

Detached shoot treatments cannot replace whole-tree assays when phenotyping for apple resistance to Neonectria ditissima

“…Significant differences with regard to resistance to European canker were observed among the potted trees when using disease incidence, with 'Royal Gala' the most susceptible while R5 trees remained lesion-free. This result confirms the general understanding that 'Royal Gala' is a susceptible accession and R5 more resistant (Gómez-Cortecero et al 2016;Amponsah et al 2017;Bus et al 2017). Our results also confirmed previous reports of the relative resistance of 'Golden Delicious' (Van de Weg 1989;Ghasemkhani et al 2015a;Garkava-Gustavsson et al 2016;Gómez-Cortecero et al 2016;Bus et al 2017), but the relative resistance of 'Sciearly' potted trees was unexpected, since this accession is known to be more field-susceptible to European canker than 'Royal Gala' in New Zealand (Bus et al 2017).…”

“…The accessions comprised of: four common apple (Malus x domestica) cultivars 'Royal Gala' , 'Braeburn' , 'Golden Delicious' and 'Sciearly'; a common apple rootstock 'Malling 9' ('M9'; M. pumila); and a crab apple 'Robusta 5' (R5; M. baccata x M. prunifolia), which is known to be highly resistant to canker (Gómez-Cortecero et al 2016;Bus et al 2017).…”

“…Breeding for resistance to European canker will result in easier, cheaper and more sustainable management of tolerant trees, but this is a lengthy process. The use of detached shoots as a fast-phenotyping method rather than whole trees would speed up this process, but in previous research, results of detached-shoot tests have differed from pottedtree tests (Garkava-Gustavsson et al 2016) and from previously reported field susceptibility (Gómez-Cortecero et al 2016). In this study, disease development in different detached shoot methods was compared with that in potted trees with the aim to improve the detached shoot test.…”

“…It is caused by the fungus Neonectria ditissima and can result in the death of buds, shoots, spurs and branches (Zeller 1926;Swinburne 1975;Weber 2014). All modern apple cultivars are susceptible to this disease, but they vary in susceptibility (Ghasemkhani et al 2015a;Ghasemkhani et al 2015b;Gómez-Cortecero et al 2016). Recently, a quantitative trait locus (QTL) for a resistance gene to this disease was identified in the crab apple cultivar 'Robusta 5' (V.G.M.…”

The mitigation effects of exogenous melatonin on replant disease in apple

“…Although the overall bacterial composition was similar among samples, the distribution of each phylum or group varied. Healthy apple trees and a more diverse soil microbial community appear to be tightly coupled, possibly giving rise to metabolic differences in parallel substrate processing in soils . Our results indicated that more unclassified species occurred in healthy soil and melatonin‐treated replant soil, which was reflected by their higher diversity indices.…”