The association between water stress and susceptibility of Quercus cerris to the fungal parasite Hypoxylon mediterraneum was studied in field-grown trees and greenhouse-grown seedlings. Susceptibility of Q. cerris to the fungus, expressed as extension of wood discolorations, increased in field-grown trees during drought and increased in greenhouse-grown seedlings in response to limited water supply. In both seedlings and trees, spread of the fungus in the vascular system was higher when leaf water potentials were low than when leaf water potentials were high, and was significantly correlated to loss of hydraulic conductivity of xylem. The presence of the fungus in the xylem suggests that H. mediterraneum could use embolized vessels to spread in the host.