Phytophthora ramorum, the causal agent of Sudden Oak Death syndrome in California, has been recently isolated from ornamental Rhododendron sp. in Mallorca (Balearic Islands, Spain). This is the first record of this polyphagous pathogen from a mediterranean climate and vegetation area outside California. To date, it seems that the Sudden Oak Death epiphytotic is limited to the fog belt area along the Pacific Coast where special climatic conditions within the mediterranean climate pattern are produced. It is unclear whether P. ramorum could thrive in drier areas.
In order to assess the risk of spread of P. ramorum from Mallorca nurseries to natural ecosystems, as it is believed has occurred before in California, we have tested the susceptibility of species from an assortment of local flora. Inoculation trials were carried out on plants of Mediterranean evergreen oak forest and maquis-type vegetation. The abaxial side of detached leaves of Quercus ilex, Arbutus unedo, Lonicera implexa, Rhamnus alaternus, Pistacia lentiscus, Ceratonia siliqua, Smilax aspera, Olea europea, Cistus salvifolius, Rubus ulmifolius, Rosa sempervirens and Clematis flammula were inoculated with 20 µl drops containing 100 to 200 zoospores, which were placed on the laminar base, margins, center and tip of six leaves of each species. These were incubated in humid chambers at 17 º C under 12 h daily fluorescent white light. Lesion development and the formation of sporangia and other vegetative structures were checked daily after 72 h, as well as sporangia produced per unit leaf area.
All plants except Clematis flammula and Rubus ulmifolius were susceptible to the pathogen. However, there were some differences of lesion expansion, sporangial production and chlamydospore formation among the species tested. Quercus ilex developed limited necrotic lesions after nine days' inoculation but the pathogen could be reisolated. Conspicuous necrotic lesions were produced 72 h after inoculation in the other plants tested and extensive blight after nine days occurred on leaves of A. unedo, O. europea, C. siliqua, R. alaternus, P. lentiscus and L. implexa. On several hosts, abundant chlamydospores were readily formed in condensation drops on leaf lesions and also sporadically and subepidermally on the adaxial side. Sporangial density ranged from 1 to 11 sporangia mm2.
Structures produced by P. ramorum similar to coelomycete acervuli, which we believe have not been previously reported for oomycetes, have been consistently observed after two weeks' inoculation on the adaxial side of leaves of O. europea, C. siliqua and P. lentiscus. Sporangiogenous layers were formed within the hosts and dehiscence was by rupture of the overlying tissue; slimy sporangial masses were liberated. The term "sporangiomata" is proposed to describe these structures.
These preliminary studies indicate a significant susceptibility of some widespread Mediterranean plants to P. ramorum. Newly morphological characters, herein reported (aerial chlamydospores and sporangiomata) give new insights on how this pathogen might become established in drier environments.
