In vitro ecology of Seiridium cardinale and allied species: the effect of solute stress and water potential on fungal growth

Defining the potential implications of global climate change on Mediterranean forest ecosystems requires a basic knowledge on the ecology of fungal pathogens under conditions that would stress host plants. The Mediterranean cypress (Cupressus sempervirens)-Seiridium spp. pathosystem represents an important case study. In the last century, epidemics of cypress canker have killed historical plantations and the decades-long host resistance will probably break down in the future as a result of both host and pathogen adaptation to increasing temperature and decreasing summer precipitation. In this study, the effect of osmotic water stress on mycelial growth of Seiridium cardinale, S. unicorne and S. cupressi in culture was examined and compared to that of Diplodia cupressi, which is a pathogen of cypress known to be favoured by host water stress. Growth responses were evaluated on potato sucrose agar amended with KCl or NaCl to give water potentials in the range of -0.34 to -15 MPa. Mycelial growth decreased with decreasing water potential and ceased at -15 MPa, although the mycelium remained alive. Histochemical analysis conducted on S. cardinale grown at -12 MPa revealed melanization and thickening of hyphal walls, in addition to abundance of lipidrich organelles. These results suggest that the three Seiridium spp. might survive drying cycles in cypress wood, but their tolerance is different. Successful survival strategies may partly result from changes in mycelium structure. Furthermore, S. unicorne was positively stimulated by a water potential of -3 MPa, suggesting that it may have high adaptive potential for life in a drier Mediterranean ecosystem, which is predicted to occur under scenarios of global warming.