WP 3

Phenotyping of apple canker susceptibility in apple cultivars

WP leader: SLU
Background

Apple canker is a severe problem in apple production in many countries but especially in Northern Europe where the damage is aggravated by winter injuries. Even when applying fungicides and good horticultural practices, canker damage occurs almost yearly in nurseries and orchards. To date, complete resistance is not known in apple. Instead, apple cultivars apparently display a continuous variation for partial resistance to the disease. This partial resistance may be under the control of a limited number of major QTL (quantitative traits loci), but information about its inheritance is very limited.

Several methods for artificial inoculations have been developed to quantify resistance to apple canker: inoculation of leaf scars on trees, wound inoculations on trees and wound inoculations on cut shoots. Two studies have recently been carried out at Balsgård, involving cut one-year-old shoots (bud sticks) as well as planted two-year old trees. The former method yields quick but crude results whereas the latter, although more informative, requires already propagated trees and therefore becomes time-consuming and expensive.

In this project, we mainly use cut shoots to identify comparatively resistant cultivars. This data can then be used to select the best cultivars for additional, more sensitive inoculation methods with trees propagated on rootstocks.

The obtained data will inform growers about susceptibility to canker, but – more importantly – will provide plant breeders with information about putative parents for making crosses. In addition, data on a separate but closely tied-in project on pedigreed germplasm screened by inoculations of grafted trees will be delivered to the international, EU-funded Fruitbreedomics project and used for analyses of the genetics of resistance to apple canker, thus enabling more efficient plant breeding efforts in the future.

Tasks

Detached apple shoots are inoculated with Neonectria spores in the spring at SLU and Graminor AS. Tested material include local germplasm, cultivars used in plant breeding and advanced selections from the breeding programs. The resulting damage is evaluated as lesion length measured with a digital caliper at regular time intervals. Sum of the values for lesion length is calculated for each assessment time-point and thus a disease-progression curve is obtained. Area under disease curve (AUDC) is then calculated and used for the statistical analyses. In addition, young potted trees of promising selections and reference cultivars are inoculated as above at SLU. 

Genetic variation and breeding system of the Neonectria fungus is evaluated using DNA markers on single-spore isolates obtained from orchards in Sweden and, for comparison, also Belgium where trees for commercial plantations in Sweden are being propagated.

Expriments with qPCR-based quantification of fungal biomass are undertaken on several apple cultivars to determine the feasibility of using this method as a means of predicting level of resistance against Neonectria.

Crosses are conducted using some of the cultivars hitherto shown to carry unusally high levels of resistance, e.g. Prairifire and Stølen, in order to produce valuable new germplasm for the breeding programs and to provide material for future genetic analyses.