Abstract: “Oak decline” is a complex syndrome affecting oak stands worldwide. Thephenomenon, reported early in the 1900’s, became a serious ecological problem starting from the70’s. In recent years, it has become more and more evident that oak decline is the result of acomplex interaction between a plurality of disease agents and environmental stresses (e.g.prolonged drought). In Mediterranean climates, hot and dry summers predispose trees to waterdeficit, a condition that is sufficient to shift the behaviour of some endophytes towardspathogenicity. The cold season, on the contrary, during which time rainfall is concentrated, ismore favourable to oomycetes for their attack and dispersal. Here we report on the isolation offungal endophytes and oomycetes from the same trees in declining stands and on some laboratorytests carried out to individuate optimal growth conditions for these agents. Our preliminaryfindings suggest a possible synergistic action between endophytes and oomycetes in incitingdisease and inducing plant dieback.