Comparative aggressiveness of Phytophthora spp. to Mediterranean oaks

Abstract: Since 2009, an increasing decline of Mediterranean oak forests and its association with several Phytophthora spp. has been reported in Sardinia. Although some of these species have been shown to be involved in oak decline in central Europe, their role in the aetiology of Mediterranean oak decline is little understood. Therefore, this study was aimed to assess the comparative aggressiveness of seven Phytophthora spp. in infecting roots and mature bark of Quercus ilex, Q. pubescens and Q. suber. Roots of 2-year-old oak seedlings of these oak species were inoculated through soil infestation and the reduction of the root system assessed after 4 months. Phloem tissues of mature trees were inoculated with plugs of Phytophthora mycelium, incubated at 20 °C for five weeks and lesion area measured. On all inoculated seedlings, P. cinnamomi was the most aggressive species, capable of causing extensive lesions and cankers on the main roots. Quercus ilex was the most susceptible species, whereas Q. pubescens showed to be more resistant to all Phytophthora species. Interestingly, P. quercina, P. psychrophila and P. uliginosa, also proved their pathogenicity towards Q. ilex, Q. pubescens and Q. suber, causing a significant reduction of the root system. Phytophthora cinnamomi and P. citrophthora were the most aggressive species in colonising the inner bark of all three oak species. Phytophthora psychrophila and P. quercina were less aggressive and did not differ statistically from the control. These results suggest the direct involvement of several Phytophthora species in the decline of Mediterranean oaks.