In the summer of 2016, extensive decline, and mortality of multiple species of Mediterranean shrubs and trees were recorded in a park in Athens (Greece). The progressive crown decline of shrubs and trees was occasionally associated with collar and stem bleeding cankers, and root necrosis. Phytophthora nicotianae was found to the responsible for such decline in the park. The possible drivers of this extensive decline were investigated and discussed with a specific focus on pathogen invasiveness and site invasibility. The pathogen was probably unintentionally introduced from commercial nurseries. Its spread was likely favored by intrinsic traits such as polyphagy, adaptation to warm climate, potential of recombination, and reproduction, long persistence of inoculum in the soil, which determine the high invasiveness in the Mediterranean climate. A multiple linear regression model was elaborated that evidenced a significant association of the level of inoculum in the plant beds with specific site traits such as total host richness and the quote of susceptible hosts. Furthermore, a multivariate model evidenced that specific host taxa were the drivers of inoculum build-up. Based on the results of the present study, the strategies to reduce the site invasibility by the polyphagous P. nicotianae without sensibly affecting the park’s ornamental value must consider the choice of plant taxa and their combination in the plant beds.