Fitness characteristics of the European lineages of Phytophthora ramorum

Abstract As an introduced pathogen, Phytophthora ramorum exists as four near‐clonal evolutionary lineages, of which only EU1 and EU2 are established in the UK. EU1 has become widespread since the first findings in 2002 whereas EU2, detected in 2011, has a more limited distribution. Both lineages are epidemic in plantation‐grown larch, sporulating asexually on needles, but also causing heavy dieback and mortality. To understand whether EU1 and EU2 pose different threats to forest health, we compared their growth characteristics on agar, pathogenicity on several hosts, and sporulation on Japanese larch needles. When pathogenicity was evaluated by measuring colonization at 20 °C in mature bark (phloem) of Japanese and European larch ( Larix kaempferi and L . decidua ), English oak ( Quercus robur ), and beech ( Fagus sylvatica ), Japanese larch was the most susceptible and oak the least susceptible. On average, EU2 isolates produced significantly larger lesions than EU1 isolates in Japanese larch and oak although not in the other hosts. With tests using young saplings of Japanese and European larch, damaging bark lesions formed at both 10 °C and 20 °C, but EU2 was significantly more pathogenic at 20 °C on both hosts compared with EU1. In contrast, both lineages caused similar amounts of necrosis on inoculated leaves of rhododendron ( Rhododendron ponticum ). Moreover, EU2 isolates usually sporulated less abundantly on larch needles compared with EU1 isolates, suggesting a trade‐off in pathogenicity and sporulation between lineages. As EU2 tends to have smaller sporangia than EU1, this could also reduce the inoculum potential of EU2.