Diplodia corticola is one of the most aggressive fungal pathogens of Quercus species and is involved in the decline of Mediterranean cork oak forests and Californian oaks. Information regarding variation in virulence between strains is scarce. We hypothesize that D. corticola strains differ in virulence and consequently induce different symptoms in infected plants. To test this, infection assays were carried out on Quercus suber half-sib seedlings with seven strains of D. corticola. Visual symptoms of infection (external lesions, leaf wilting, exudation and others) were recorded in parallel with physiological and biochemical parameters. All strains were able to cause lesions but at differing levels of aggressiveness. We show that internal lesion length did not correlate directly with strain aggressiveness and this agrees with physiological parameters that should be taken into account to infer about strain pathogenicity. Infection by all strains induced an overall negative impact on the net photosynthetic rate and an increase in the oxidative stress status of plants; however, significant differences were found when the effects of different strains were compared. Results also suggest that being under optimum growth conditions, prior to and during infection, allowed plants to respond to the pathogen. At the end of the experiment, some strains of D. corticola established a latent pathogen-like relationship with cork oak. This is the first study to show that D. corticola virulence is strain-dependent.