First Confirmation of Phytophthora lateralis in Europe

Phytophthora lateralis, a pathogen of Chamaecyparis lawsoniana (Port-Orford cedar or Lawson's cypress), was confirmed in France, but isolates from Germany identified as P. lateralis or “similar to” P. lateralis proved to be P. gonapodyides. Previously, P. lateralis was known only from western North America, where it has been destructive in nurseries, ornamental plantings, and the forest since its introduction about 1920 (1). Reports from other locations have proved to be misidentifications or impossible to confirm. In France, P. lateralis was isolated and identified from C. lawsoniana on two occasions (1996 and 1998) in different parts of the country, probably stemming from a single original infestation of young, potted, greenhouse-propagated cedars in a commercial nursery. German isolates were from an old culture collection and from irrigation water in a nursery growing a wide range of woody ornamentals. Identifications were confirmed by comparison (2) with authentic isolates. P. lateralis isolates from France and Oregon formed laterally proliferating, elongated obpyriform sporangia that under the conditions of our tests could be dislodged by agitation, leaving a short pedicel. Also, brown chlamydospores formed laterally on the hyphae or terminally on a short stalk and oospores were not formed on standard media. Radial growth was about 2 mm per day. In contrast, sporangia of German isolates and known P. gonapodyides isolates were similar. They exhibited nested pro liferation, were broader than P. lateralis sporangia, and were not readily dehiscent. Some P. gonapodyides isolates, including those from Germany, formed chlamydospores, but these were nearly all catenulate and not lateral, and isolates grew faster (3 to 4 mm per day). Pathogenicity was tested by stem inoculation of C. lawsoniana. P. lateralis from France and Oregon produced lesions averaging 4.7 cm after 2 months (range 2.0 to 8.1 cm, six replicates per isolate, five isolates) while the six replicates of the two German isolates averaged 1.2- and 1.6-cm lesion lengths. Furthermore, sequences of internal transcribed spacer (ITS) DNA from French and Oregon P. lateralis isolates were identical, while sequences of German isolates were similar to P. gonapodyides (J. Duncan and D. Cooke, personal communiation). P. lateralis is a dangerous pathogen of C. lawsoniana and is also pathogenic to Taxus spp. (1), although less aggressive on this host. If established, it would be a serious threat to the widespread ornamental plantings and scattered forest plantations of C. lawsoniana in Europe. References: (1) E. M. Hansen and K. J. Lewis. Compendium of Conifer Diseases. American Phytopathological Society, 1997. (2) C. M. Tucker and J. A. Milbrath. Mycologia 34:94, 1942.