First Detection of Phytophthora ramorum Mating Type A2 in Europe

Since its original isolation in 1993, Phytophthora ramorum has become an important pathogen. Initially, it was determined to be the causal agent of a twig blight of Rhododendron spp. in Germany and the Netherlands (3). Around the same period, symptoms and mortality on oak (Quercus spp.) and tanoak (Lithocarpus densiflorus) were associated with P. ramorum in California (2), where the disease was named sudden oak death. Subsequently, P. ramorum has been detected on a wide range of forest trees and shrub species in the United States. In Europe, the pathogen has spread to many countries, primarily on nursery plants of Rhododendron and Viburnum spp., and recently, on Camellia japonica, Kalmia latifolia, Pieris formosa var. forrestii, P. japonica, Leucothoe sp., Syringa vulgaris, and Taxus baccata. P. ramorum has not been observed in European forests. P. ramorum is heterothallic, and initial in vitro mating studies on agar media suggested that only the A1 mating type occurred in Europe, while only the A2 mating type was present in the United States (4). However, an isolate collected in 2002 in Belgium (1) appears to be the A2 mating type. This isolate (CBS 110901, Centraal Bureau voor Schimmelcultures, Baarn, the Netherlands) originated from an imported V. bodnantense plant at an ornamental nursery. A hyphal tip culture (BBA 26/02) of this isolate produced no oogonia on carrot piece agar after 6 weeks in pairing tests with other Phytophthora species of mating type A2. When paired with mating type A1 of P. cambivora, P. cinnamomi, P. cryptogea, and P. drechsleri, however, oogonia were observed in all pairings within 6 weeks. The number of oogonia was low in all pairings but was highest in pairings with P. cryptogea. No oospores were produced after 6 weeks between P. ramorum isolates BBA 26/02 and BBA 9/95 (from the holotype, mating type A1), but gametangia were observed when these isolates were paired on Rhododendron sp. twigs. Normal oogonia were produced on the outgrowing mycelium when pieces from these twigs were placed on carrot piece agar. The shape and size of the oogonia produced on carrot piece agar after pairing with P. cryptogea and on Rhododendron sp. twigs after pairing with P. ramorum BBA 9/95 were similar (24 to 34 μm, mean 29.6 μm and 25 to 33 μm, mean 30.6 μm, respectively). To our knowledge, this is the first observation of P. ramorum mating type A2 in Europe. References: (1) D. De Merlier et al. Plant Dis. 87:203, 2003. (2) D. M. Rizzo et al. Plant Dis. 86:205, 2002. (3) S. Werres et al. Mycol. Res. 105:1166, 2001. (4) S. Werres and B. Zielke. J. Plant Dis. Prot. 110:129, 2003.