An emergent pathogen, phytophthora ramorum, in Irish woodlands

Abstract

Phytophthora kemoviae and Phytophthora ramorum are both recently discovered pathogens of tree species. The disease that causes P. ramorum has been given the common name of Sudden Oak Death, in the US, because the expression of symptoms appears across wide swathes of forest or woodland very near the time of death of trees. Their principal host in Europe is rhododendron, primarily Rhododendron ponticum, which is widely distributed throughout many of Ireland’s oldest oak forests. In 2003, P. ramorum was found, by the Department of Agriculture, Fisheries and Food (DAFF) on Rhododendron ponticum in the wild. Since spring 2005, sampling in this study has been earned out on susceptible foliage and in soil and watercourses from 11 susceptible forest sites in Ireland. Identification was carried out using a rapid DNA method in conjunction with morphological identification methods. Each site was sampled twice a year (2005-2008) by collecting foliage and using water and soil baits. The pathogen was identified at three sites in SW Ireland: Killarney National Park, Co. Kerry, Lauragh, Co. Kerry, and Castletownbere, Co. Cork, and was not isolated from oak trees. Genetic variation was compared between Irish forest and nursery populations of P. ramorum using eight microsatellite loci. All 93 isolates had the same multilocus genotype (MG) indicating no genetic differences between wild and nursery isolates. Up to 2008, P. kemoviae had only been reported in the UK, but has now been identified at two locations in New Zealand. Protocols have been developed that unequivocally discriminate the two pathogens from all other Phytophthora plant pathogens. Using real-time PCR, samples collected from the 11 susceptible forest sites, over the three-year period, were analysed for the presence of P. kemoviae and no positives in a total of 205 samples were found. Susceptibility analyses indicated that the Irish isolate of P. ramorum, K2, can infect the leaves of rhododendron (R. ponticum), fuschia (Fuschia magellanica), holly (//ex aquifolium), sessile oak (Quercus petraea) and the strawberry tree (Arbutus unedo). Additional studies on the long-term survival potential of P. ramorum showed that the chlamydospores of K2 are capable of surviving at temperatures of 4-20°C and in a pH range of 3-9, conditions that prevail in the soil in SW Ireland throughout the year. Extreme temperatures of -21 and 40°C and pH 2 are not suitable for the long-term survival of the pathogen.