Can choice of rootstock reduce damages caused by fruit tree canker in apple orchards?

The fungus Neonectria ditissima causes fruit tree canker, a serious desease on apple and pear. In the past years the disease has become a threat for Swedish and Northern European apple production since devastating outbreaks destroy large numbers of trees. To date, no complete genetic resistance to N. ditissima is known in apple but genotypes (scion cultivars and rootstocks) differ greatly in their level of partial resistance. Furthermore, susceptibility of a scion cultivar may be influenced by the rootstock it is grafted to. Thus we aimed to improve our understanding of genetically determined differences in resistance among rootstocks and clarify cultivar/rootstock interactions with regards to canker resistance. For that, two experiments where conducted where differences in resistance were evaluated in 23 rootstocks (including two ‘M.9’ clones) and in the four most common scion cultivars in Swedish orchard grafted to four rootstocks differing in vigour (16 rootstock/scion combinations). The trees were inoculated in a greenhouse in artificially created wounds and the symptoms were assessed seven times every second week. In the experiment on rootstocks, significant differences (p<<0.001) in resistance were observed, with ‘B.9’ and ‘P 60’ being the most susceptible and ‘G.969’ the most resistant. In the experiment on scion/rootstock combinations significant effects were found for scion (p<<0.001), rootstock (p<<0.001), and scion × rootstock interaction (p=0.008). The correlation between rootstock resistance and rootstock effect on scion resistance was non-significant (p=0.23). The new knowledge will be useful for growers, nurserymen and breeders.