LAUSR

Forests in Europe are threatened by increased diversity of Phytophthora species, but the effects of the simultaneous presence of these pathogens on trees and the ecological consequences of their coexistence are unknown. This study aimed to explore variation in early survival of Quercus ilex to Phytophthora infections and assess interactions between Phytophthora species when trees are co-infected separately in time. Three Phytophthora species (P. cinnamomi, P. gonapodyides and P. quercina), seeds from 16 populations of Q. ilex (from both ballota and ilex subspecies) and two infection times were used as sources of variation in two experiments. The influence of the Phytophthora species, Q. ilex subspecies and populations on plant germination and survival were analysed using generalised linear mixed models and survival analysis techniques. Germination rates were not influenced by Phytophthora spp. (P = 0.194) but by the subspecies and populations of Q. ilex (P < 0.001). In Phytophthora infested soils, acorns of Q. ilex ssp. ilex germinated at higher rates than acorns of Q. ilex ssp. ballota. In contrast, plant survival was strongly influenced by Phytophthora species (P < 0.001), but not by the subspecies and populations of Q. ilex. Reduced and delayed seedling mortality were observed if a less virulent Phytophthora species infected plants prior to infection by a more virulent Phytophthora species, e.g. plants infected first by P. cinnamomi died at higher rates and earlier than plants infected first by P. gonapodyides and P. quercina (P = 0.001). The results help to explain oak decline syndrome and the lack of natural and artificial regeneration of Q. ilex forests. The lack of interspecific variability of early survival to Phytophthora spp. discourage direct sowing as a strategy in artificial reforestation programmes. Large, thick seeds that give plants rapid growth are advantageous traits if soils are infested with Phytophthora spp. This article is protected by copyright. All rights reserved.