The first findings of Phytophthora ramorum in Europe date from 1993 (The Netherlands) and Germany (1994). Sudden Oak Death, also caused by P. ramorum, was observed in California from 1995. Most probably, the fungus was eparately introduced both in Europe and in the USA. In California, P. ramorum has since killed large numbers of coast live oaks and tan oaks, and a number of new host plants representing several plant genera has been found during the last two years. However, in Europe, P. ramorum has only inflicted damage on Rhododendron spp. and Viburnum spp. plants so far.
It is not clear why P. ramorum has not been associated with oak death in Europe. Climatic differences would not seem the most likely explanation, because the fungus does spread on Rhododendron and Viburnum species. European white oak species, in north-western Europe mainly Quercus robur and Q. petraea, might be resistant to P. ramorum, in contrast to native red oaks in California. Alternatively, the pathogenicity of the American population of P. ramorum might differ from its European counterpart. The latter hypothesis is supported by apparent differences in mating type (A1 versus A2), and minor molecular differences between both P. ramorum populations (Bonants and Kroon, unpublished; Ivors and Garbelotto, unpublished). However, such small differences could also be due to a founder effect, with two narrow and slightly distinct lineages having been introduced independently in Europe and California.
We tested whether European white oaks are susceptible, and also whether European P. ramorum isolates differ in pathogenicity from American ones. In a glasshouse experiment under strict quarantine conditions, the type isolate (CBS 101553) and isolate O4 from Q. agrifolia, California were inoculated on young plants of Q. robur, Q. rubra, Fagus sylvatica, Lonicera periclymenum, Vaccinium myrtillus, Viburnum (x) bodnantense 'Dawn', and Rhododendron ponticum. Isolate O4 had been generously made available by Matteo Garbelotto (Berkeley) and Dave Rizzo (Davis). Two treatments were applied, viz. dipping the foliage into a suspension of sporangia and mycelial fragments of the fungus, and placing discs of agar bearing mycelium on wounded stems. As a control, foliage of plants was dipped into water, or pure agar discs were placed on wounded stems, respectively. Plants were maintained at 18&Mac176;C and 100% humidity (after 2 weeks 80% humidity) for three months. As expected, Rhododendron ponticum plants were highly affected, developing severe leafspots and stem lesions after 1 week. Plants of the European wild blueberry, V. myrtillus, died within 4 weeks. Plants of F. sylvatica and Q. rubra, infected on wounded stems, showed a severe dieback of twigs after 4 weeks. Plants of Q. robur seemed free of damage, as was the case with V. x bodnantense. Plants of L. periclymenum also remained unaffected. All control plants remained healthy. The European P. ramorum isolate was equally pathogenic as the American one.
Our results suggest that European Q. robur is not susceptible to P. ramorum; however, European beech and Q. rubra, also common indigenous respectively exotic species in European forests, proved to be susceptible. The importance of stem wounding for establishing infections suggests that the latter two species may have some field resistance. The lack of damage on V. (x) bodnantense may indicate that infection occurs by root infection, in line with the formation of basal cankers on this host and the absence of leaf infection in practice. More research is required to test whether or not native European oak species and other trees found not susceptible in this experiment can be infected through the roots or stem base. Further experiments on European forest tree species, lane trees and hardy ornamentals will be conducted in the coming year.
