Progress Report on the Evaluation of the Susceptibility of the Holm Oak (Quercus ilex) Forest Ecosystem to Phytophthora ramorum

Eduardo Moralejo & Enrique Descals, Institut Mediterrani d’Estudis Avançats, IMEDEA (CSIC-UIB), c/ Miquel Marqués 21, 07190 Esporles, Majorca, Spain; (34) 971 611828; vieaemr@uib.es

In preliminary studies on the susceptibility of plant members of the holm oak (Quercus ilex) forest, detached leaves of several woody species were highly susceptible when inoculated with zoospore suspensions of local isolates of Phytophthora ramorum (Moralejo & Hernández 2002). Since then, there have been reports of natural infections by P. ramorum on leaves and shoots of Q. ilex in the UK, and new findings on Viburnum tinus in nurseries across Europe. Taken together, this indicates that at least some areas of Mediterranean vegetation with a prevalence of the Arbutus unedo-Viburnum tinus-Quercus ilex association should be closely monitored. Yet, there are so far no outdoor infections in the Mediterranean basin.

For having a more complete picture of what might occur at the plant community level, we are further testing the susceptibility of fruits, leaves and twigs of shrubs and vines of the understory, as well as logs of Q. ilex to different isolates of P. ramorum in vitro. The sporangial production on leaf lesions is being assessed to identify potential major sources of inoculum. Available protocols of the European Project RAPRA, either for testing the susceptibility of tree and non-tree species or for determining sporulation capacity are being used. Leaf lesion areas developed 4 and 7 days after inoculating are measured in cm2, and the sporangial production is rated as number of sporangia/ leaf lesion unit (cm2). The length of the twig lesions is measured 10 days after wound inoculating. For the log trials, the necrotic area is calculated in cm2 ca. 40-50 day after wound inoculating.

Results of the ongoing research on susceptibility of fruits (9 species), leaves and twigs (7 species), and logs (Q .ilex and Pinus halepensis) to five P. ramorum isolates will be presented. Collected data are being analyzed to determine (i) whether isolates of P. ramorum differ in pathogenicity, (ii) to rank the susceptibility of different organs of each potential host, (iii) to identify which hosts might act as major sources of inoculum, (iv) to calculate the latent period, i. e. time from infection to the first appearance of sporangia, and (v) to determine the capacity of P. ramorum for invading the bark of Q. ilex and P. halepensis.

References:

Moralejo, E. & Hernández, L. (2002) Inoculation Trials of Phytophthora ramorum on Detached Mediterranean Sclerophyll Leaves. I Sudden Oak Death Science Symposium, The State of our Knowledge. Monterey, CA, December 15-18.

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