Effects of water stress on the endophytic mycobiota of Quercus robur

Oak decline is expanding throughout European and North American forests, in connection with increasing drought periods. The role of endophytic communities in oak decline is almost unknown, considering that these communities may harbour latent pathogens promoted by host weakness. This experiment was designed to assess if and in which extent endophytic mycobiota of adult English oak (Quercus robur) trees are affected by water stress. In a typical oak plain forest of north-western Italy, two square plots were established each including 25 oak trees. In one plot a polyethylene layer was placed for rainfall flow away to occur and, consequently, to induce water stress in trees. Predawn Water Potential (PWP) and Midday Water Potential (MWP) were used as water stress indicators and measured constantly. Three seasonal samplings (spring, summer, and fall) were carried out to isolate fungal endophytes on herbaceous and woody tissues of trees from both plots. Twenty-five taxa were isolated, most of which were ascomycetes and anamorphic fungi. Tubakia dryina and Phomopsis quercina were the most frequent fungi in annual shoots. The frequency (F) of T. dryina was significantly lower in water-stressed than in control trees (2 test, P<0.05) both in summer and in autumn, when water stress was higher. The F of P. quercina increased over time. The F of Monochaetia monochaeta in annual shoots resulted significantly higher in control trees than in water-stressed trees starting from the summer. The correspondence analysis discriminated shoot communities of water-stressed trees from those of control trees, because of the different ratios between T. dryina and P. quercina.