The progress of colonization of ash stems from ascospore inocula of Hymenoscyphus fraxineus was examined by light and electron microscopy. The main aim of the study was to characterize the cytology of the biotroph to necrotroph transition during lesion formation. Following direct penetration into epidermal cells, the fungus produced intracellular hyphae that invaded up to five cells before plant cells died. A lack of close attachment between the hyphal cell wall and plant cell membrane was revealed by plasmolysis of epidermal cells. Plant cells died at the centre of the infection but hyphae at the edge were typically found in living plant cells even around large lesions. During biotrophic invasion, the cytoplasm of penetrated plant cells showed very little response despite the plant cell membrane being in direct contact with the fungal cell wall. Before plant cell death, dark staining of the cytoplasm and proliferation of small vesicles was noted, but organelles retained normal ultrastructure. Dead plant cells contained dark brown, osmiophilic droplets. Penetration between epidermal or collenchyma cells was usually targeted to shared pits and involved constriction of hyphae. The transition to necrotrophy was not associated with a clear change in hyphal morphology. Biotrophic intracellular hyphae contained dense cytoplasm but hyphae in dead plant cells were more vacuolated. Remarkably little plant cell wall degradation was observed despite the fungus penetrating up to 18 cells deep into stem tissue. Features of the development of the ash dieback fungus are compared with other hemibiotrophic pathogens.

Bacterial and fungal endophytes may help their host in terms of improved tolerance to abiotic and biotic stresses and enhanced growth. European apple canker, caused by Neonectria ditissima, is widespread in apple growing regions. Infection by N. ditissima occurs through artificial or natural wounds, including leaf scars, picking wounds, and pruning cuts. Using F1 progeny trees in an experimental orchard derived from a cross between a canker susceptible genotype and a canker tolerant/resistant genotype, we assessed the influence of genotype of both bacterial and fungal endophyte communities in apple leaf scars, and determined correlations of endophytes with canker development. All trees were artificially inoculated with a N. ditissima isolate post-planting. Specific components of apple endophytes as well as a number of individual fungal/bacterial groups in leaf scars were partially genetically controlled by host genotypes. Several bacterial groups were significantly correlated with canker-related traits, mostly positively associated with canker tolerance. A few fungal groups may facilitate canker development whereas others may compete with canker. However, most of these microbial groups could not be identified to the species level with confidence; even for those groups which could be assigned to the species level there is insufficient knowledge about their ecological characteristics in relation to plants. The present results may be used to inform further research using biocontrol to manage N. ditissima and breeding for resistance.

No abstract

European apple canker, caused by Neonectria ditissima, is a damaging disease of apple in many production regions worldwide. The pathogen infects apple trees through artificial or natural wounds. The most damaging phase of the disease is that cankers on main stems post-planting, most likely originating from infection in nurseries, can result in tree death in young orchards. Apple cultivars differ in their responses to the pathogen, which may be additionally affected by specific site factors. An experiment was conducted to study i) the susceptibility of seven cultivars to N. ditissima at three sites and ii) the effects of cold storage duration prior to planting on subsequent development of both main stem and peripheral cankers. Planting date had significant (albeit minimal effects) on the development of peripheral cankers only. Canker development differed greatly among the three sites and between the tested cultivars, with ‘Grenadier’ and ‘Golden Delicious’ being most resistant at all sites. The relative performance of cultivars in terms of canker development was generally consistent across the three sites. Nevertheless, the interaction between cultivar and site was still statistically significant for the development of main stem cankers, indicating that some site-specific factors may interact with cultivars to affect development of latent infections. Given the close proximity of the three sites (similar climatic conditions), the results indicate that further research is needed to investigate the effects of soil characteristics on canker development post-planting.

Apple canker, caused by Neonectria ditissima (Tul. and C. Tul.) Samuels and Rossman, is a major disease of apples (Malus domestica) worldwide. N. ditissima infects through natural and artificial wounds. Infected wood develops canker lesions which girdle branches and main stems causing reduced yield and tree death. N. ditissima is difficult to control; removal of inoculum (cankers) is expensive and therefore seldom practiced, whilst effective chemical products are being banned and no biocontrol products have been found to be effective against N. ditissima. This study used cues from a previous apple endophyte community analysis to isolate and test fungal endophytes belonging to the genus Epicoccum as potential endophytic biocontrol agents. Epicoccum nigrum B14-1, isolated from healthy apple trees, antagonised N. ditissima in vitro and reduced the incidence of N. ditissima infections of leaf scars by 46.6% and pruning wounds by 5.3% in field conditions at leaf fall. Autumn application of B14-1 conidia increased E. nigrum abundance in apple tissues at 10–20 days post-inoculation by ca. 1.5×, but this returned to control levels after one year. E. nigrum B14-1 did not cause detrimental effects on apple foliage, buds, fruit, or growth and could therefore present a new biocontrol agent to manage N. ditissima in commercial apple production.