Phytophthora ramorum is the cause of Sudden Oak Death in the USA and also infects many ornamental shrub species in both North America and Europe. In addition, it is now causing widespread disease in UK commercial larch plantations. The epidemiology of P. ramorum including sporulation potential, pathogenicity, ability to persist and the changing threat posed by the recently discovered EU2 lineage of the pathogen were investigated. Results indicated that all three species of commercially grown larch in the UK are not only able to support prolific sporulation of P. ramorum (approximately 960 sporangia per cm2) but are similarly vulnerable to bark colonisation, resulting in extensive dieback and mortality. Sporulation of P. ramorum on larch foliage exceeded that of all known sporulating hosts, including Californian bay laurel (approximately 77 sporangia per cm2), which drives epidemics in North America. Sporulation potential on foliage and larch bark susceptibility varied significantly over the growing season and with genotype. Sporulation on larch foliage was highest in summer but lower in spring and autumn. In contrast, susceptibility of larch bark was highest in spring and decreased in late summer. Asymptomatic infection of foliage occurred early in the year with symptom development mainly seen on old foliage and late in the year. Phytophthora ramorum also persisted in the litter layer but not the soil for up to two years after removal of infected Japanese larch in south-east England. In comparison with the widespread EU1 lineage, the EU2 had a faster grow rate over 2.5-29°C and was significantly more pathogenic to Japanese larch, European larch and English oak bark. However, isolates of the EU2 lineage produced significantly lower inoculum loads than the EU1 suggesting a trade-off between pathogenicity and sporulation. The implications of the aggressive nature of P. ramorum infection on commercially grown larch, its persistence on infested sites and the new established adaptively different EU2 lineage are discussed in relation to the future of disease management in the UK.