Clive M. Brasier1, Joan Rose2, Susan A Kirk2, and Joan F Webber2
P. ramorum has not been found on trees in Europe. In 2001, a series of pathogenicity tests was initiated with the aim of assessing: (i) the risk posed by P. ramorum to native UK/European Quercus, Fagaceae and other European trees; (ii) the comparative risk posed by the North American (NA) and European (EU) P. ramorum populations; (iii) the comparative susceptibility of Quercus rubra. The experiments (in quarantine chambers) employ the 'log inoculation method'; (Webber & Hedger 1986), later adapted for assessing susceptibility of Quercus, Alnus and other tree species to Phytophthora bark pathogens (Brasier & Kirk 2001). As this method involves wound inoculation of freshly cut logs, it tests the pathogen’s potential to attack the phloem and cambium after bark entry. It provides consistent linear measurements for assessing genetic differences in aggressiveness between wild isolates or progenies. After a 5-6 week test at 20°C, unattacked bark is usually still 'green.' About 8 replicate inoculations per isolate, randomised overall, are used. In UK, tests are usually best conducted during summer (see Brasier & Kirk, 2001).
Three experiments were conducted in 2001 and another five in 2002, on a total of twenty-three hosts. Common European deciduous oak (Quercus robur) was included in seven experiments; and eastern American red oak (Q. rubra) in three. Most tests have involved at least two EU and two NA isolates of P. ramorum. Control isolates of P. cinnamomi, P. cambivora and alder Phytophthora have behaved as predicted. Results so far are summarised here. (i). Bark of the following species appears more susceptible to P.ramorum: Q. ilex, Q. cerris, Q. rubra, Fagus sylvatica, Picea sitchensis, Pseudotsuga menzeisii, Chamaecyparis lawsoniana and Rhododendron ponticum,. (ii). The following appear less susceptible: Q. robur, Q. suber, Castanea sativa, Betula pubescens, Aesculus hippocastanum, Prunus laurocerasus, Acer pseodoplatanus, Alnus glutinosa, Sequoia sempervirens and Taxus baccata. (iii). The following appear resistant: Tilia cordata, Carpinus betula, Populus sp., Ulmus procera and Fraxinus excelsoir. Q. robur was in the 'less susceptible' category in all seven experiments. Q. cerris was 'more susceptible' to two EU isolates but 'less susceptible' to two NA isolates. Further tree species will be tested in future. 'More susceptible' hosts will be investigated under different test conditions.
In culture North American P. ramorum isolates were, on average, slower growing at 20°C than European isolates. NA isolates were also more phenotypically variable, comprising both a wild-type form and a range of non wild-type forms; whereas EU isolates were very uniform and wild-type. In a direct comparison 15 EU isolates were, on average, more aggressive than 15 wild-type NA isolates in bark of both Q. rubra (mean lesion areas 31.3 and 22.3 cm2) and Q. robur. However there was a large range overlap, some NA isolates being about as aggressive as the most aggressive EU isolates. Further NA/EU comparisons are in progress.
These studies complement work by UK Central Science Laboratory on foliage susceptibility of woody ornamental and hedgerow plants.
References
Brasier, C. M. & Kirk, S. A. (2001). Plant Pathology 50, 218-229.
Webber, J. S. & Hedger, J. (1986). Transactions of the British Mycological Society 86, 93-101.
