First Report of A1 Mating Type of Phytophthora ramorum in North America
Authors: Hansen E. M. , Reeser P. W. , Sutton W. , Winton L. M. , Osterbauer N. 
Published: May 2007

Phytophthora ramorum is known in Europe and the western United States (1). In Europe, it is found in nurseries and landscape plantings. In the United States, it has been confined to coastal forests, and in California, it is found in a few horticultural nurseries. All European isolates tested have been A1 mating type, while all North American isolates were A2 mating type (2). Amplified fragment length polymorphism markers also indicated that the populations on the two continents are distinct, and nearly all North American isolates are from one clone (Kelly Ivors, unpublished). In June 2003, P. ramorum was isolated from diseased Viburnum and Pieris spp. cultivars from a Clackamas County nursery in northern Oregon and diseased Camellia sp. cultivar from a Jackson County nursery in southern Oregon. Representative isolates were submitted to the American Type Culture Collection, Manassas, VA. As part of the effort to determine the origin of these new infestations, we tested the nursery isolates for mating type. Seven Oregon nursery isolates, three Oregon forest isolates (from the predominant North American clone), and two European isolates were paired. Agar plugs from 3-day-old colonies were placed in close proximity on carrot agar plates, and then the plates were examined for oogonia after 3 and 10 days as advised by C. M. Brasier (personal communication). Oogonia and antheridia typical of P. ramorum (2) formed when isolates from the Clackamas County nursery were paired with the Oregon forest isolates and also when isolates from the Jackson County nursery were paired with the European isolates. Gametangia also formed in pairings between Oregon forest isolates and European isolates, but not in any other combinations. We developed polymerase chain reaction (PCR) primers for four microsatellite loci and determined allele sizes for the same set of isolates (unpublished). Microsatellite alleles of the Clackamas County isolates were identical to the European tester isolates, and alleles of the Jackson County isolates were identical to the Oregon forest isolates. These results indicate that the recent Oregon nursery infestations are of separate origins. The Clackamas County isolates are A1 mating type and have microsatellite alleles like the European testers, but according to shipping records, the nursery has received no host nursery stock directly from Europe. However, host nursery stock has been received from a Canadian nursery. The Jackson County isolates are of A2 mating type and have microsatellite alleles like the forest isolates of Oregon, which is consistent with the reported origin of these plants from a California nursery. These preliminary microsatellite results need to be validated against a larger isolate set but are congruent with the mating type results. The Oregon nursery infestations highlight the dangers of unregulated or underregulated transport of host nursery stock from infested areas to noninfested areas. All host plants from infested nursery blocks at the affected Oregon nurseries have been destroyed by incineration, and a monitoring program has been implemented. Other host nursery stock on site has been taken “off-sale” pending verification that it is disease free, per the United States Department of Agriculture, APHIS requirements.

References: (1) J. M. Davidson et al. On-line publication. doi:10.1094/PHP-2003-0707-01-DG. Plant Health Progress, 2003. (2) S. Werres et al. Mycol. Res. 105:1155, 2001.