Water Stress and the Development of Cankers by Diplodia mutila on Quercus robur

Some experiments were conducted (in duplicate) to determine the effects of the osmotic potential on the growth of Diplodia mutila, and the effects of the pre-dawn water potential on the development of cankers by D. mutila on Quercus robur. Mycelium of D. mutila was grown on liquid medium enriched with various amounts of polyethylene glycol so as to give osmotic potential values of -0.35, -1.18, -1.75, -3.00 and -3.98 MPa. At the two lowest MPa values (-3.00 and -3.98 MPa) D. mutila growth was slower in relation to the fresh weight and the dry weight of the mycelium. To determine the pre-dawn water potential 30 3-year-old Q. robur seedlings were inoculated with D. mutila mycelium. Water-stressed seedlings had lower pre-dawn water potential values (MPa -2.92) and longer cankers than the unstressed seedlings (MPa -0.79). It is concluded that Q. robur, a highly hygrophilous oak, suffers from water stress to such an extent that its structural and chemical defences are overcome and the seedlings are seriously damaged. A similar situation is observed in nature, where frequent and prolonged droughts may induce states of water stress that account for the spread and the incidence of D. mutila in Italy.