The fungus Neonectria ditissima causes European canker of apple. Apple accessions vary in their resistance responses, but to date no immunity to European canker has been demonstrated. Similarly, pathogen strains vary in aggressiveness, but no race structure has been reported for this pathogen. To study the host-pathogen interactions, 14 accessions were inoculated with between four and ten single-spore isolates.
Abundant variation in both isolate pathogenicity and host susceptibility was observed, with significant isolate and accession effects. In a reduced data set, three isolates with very low pathogenicity on all accessions and accessions that were tested against low numbers of isolates were removed. In this data set of six accessions inoculated with four to seven isolates, the interaction between accession and isolate effect was significant.
A strong indication of a differential interaction was observed in a partially resistant progeny (AJ102) from an ‘M9’ x ‘Robusta 5’ family, where four isolates caused few or no lesions, while a fifth isolate (RS509c) caused lesions in 40% of the inoculated wounds. Isolate RS509c was less aggressive (16% incidence) on a more susceptible accession from the same family (AJ80), on which six other isolates were aggressive (30 to 60% incidence). Another accession (AJ167) from the same family developed few lesions with four isolates, but did develop lesions when inoculated with three other isolates. One of the three isolates that was aggressive on AJ167, caused few lesions on AJ102, while another was RS509c, demonstrating differential accession-isolate interactions.
Our research contributes to an improved understanding of the host-pathogen interactions in the Malus-N. ditissima pathosystem, which may include differential interactions.