Ash dieback is a tree disease caused by the fungal pathogen Hymenoscyphus fraxineus. Since its introduction into Europe, it has caused widespread and significant losses of the European ash, Fraxinus excelsior. Inoculations of F. excelsior with a low virulent H. fraxineus isolate was assessed as a promising method for reducing symptoms associated with ash dieback, presumably by triggering systemic induced resistance. Two strains of H. fraxineus were chosen based on observations of high and low in planta virulence. Crude extracts obtained from cultures of the highly virulent strain were more phytotoxic in a leaf puncture assay than ones obtained from the low virulent strain. UHPLC-DAD-MS/MS data identified the phytotoxin viridiol and the potential phytotoxin hyfraxin A in both cultures. However, the production of these compounds in vitro did not correspond with virulence in planta. To test the effects of priming, saplings of F. excelsior were first inoculated with the low virulent strain and subsequently with the highly virulent strain. On average, necrosis expansion on the stems was reduced by 54% in primed saplings at the end of the monitoring period of 14 weeks, thus providing proof of concept for priming. These results contribute to our understanding of a possible integrated biological disease control approach for increasing resistance in saplings and reducing potential damages associated with pathogens, particularly during nursery propagation, out-planting and through the establishment phase. We discuss the results in the context of relevant literature and summarise the limited availability of literature on priming and underlying principles in trees.