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First Report of Dieback and Leaf Lesions on Rhododendron sp. Caused by Phytophthora hedraiandra in the United States

Surveys for Phytophthora ramorum in Minnesota nurseries revealed the presence of P. hedraiandra de Cock & Man in't Veld and several other Phytophthora species but not P. ramorum. Symptomatic leaf and stem tissues from diseased Rhododendron and Quercus species were cultured on PARP, a selective growth medium for Phytophthora (3). The Phytophthora isolates obtained were later identified by sequencing the internal transcribed spacer (ITS) region of the rDNA and comparing the sequences with those in GenBank using BLAST searches (1). The ITS sequences of six cultures (GenBank Accession Nos. DQ139804-DQ139809), isolated during 2003 from various Rhododendron cultivars exhibiting leaf lesions and shoot dieback, showed 100% identity with the ITS sequence of P. hedraiandra (GenBank Accession No. AY707987) (2). This is a recently described pathogenic species from the Netherlands responsible for causing leaf spots on Viburnum spp. Since the ITS sequence of P. hedraiandra differs little from that of P. cactorum (2), we verified one isolate to be P. hedraiandra by sequencing the mitochondrial cytochrome c oxidase subunit I gene (cox1) (GenBank Accession No. DQ139810). Comparison of this sequence with the P. hedraiandra voucher specimen in GenBank (Accession No. AY769115) showed 99% identity, which was the closest match. Reproductive structures were measured on V8 juice agar. The average oogonium diameter for three isolates was 29 μm with a range of 26 to 32 μm, while the average antheridium length was 13 μm (11 to 15 μm). Sporangium length and width averages on crushed hemp seeds were 32 μm (28 to 36 μm) and 26 μm (21 to 30 μm), respectively, with the average length to width ratio of 1.25 (1.23 to 1.29). Pathogenicity tests on Rhododendron cv. Mikkeli were carried out using three of our P. hedraiandra isolates. Spore suspensions of 2 × 10⁴ zoospores per ml were used to mist-spray shoots of five, 3-year-old plants for each isolate. Five controls were mist sprayed with water. After inoculation, plants were placed in plastic bags in a dark growth chamber (22°C) for 7 days and then moved to a greenhouse. Leaf blotches and shoot dieback were apparent 5 days after inoculation, and P. hedraiandra was reisolated from those symptomatic tissues and identified by an exact match of the ITS sequence. Necrotic areas lengthened from the shoot tips to the main stems of the plants while expanding into petioles and leaves. No symptoms were observed on control plants. To our knowledge, this is the first report of P. hedraiandra in the United States as well as the first report of Koch's postulates performed with P. hedraiandra on Rhododendron cv. Mikkeli. The significance of this disease to other woody plants in nurseries or the landscape is unknown, and further study is needed to determine the host range and extent of the disease that may occur from this introduction.

References: (1) S. F. Altschul et al. J. Mol. Biol. 215:403, 1990. (2) A. W. A.M de Cock and C. A. Lévesque. Stud Mycol 50:481, 2004. (3) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996.