Detection of Phytophthora ramorum Blight in Oregon Nurseries and Completion of Koch's Postulates on Pieris, Rhododendron, Viburnum, and Camellia
Authors: Parke J. L. , Linderman R. G. , Osterbauer N. K. , Griesbach J. A. 
Published: May 2007

Phytophthora ramorum, the cause of sudden oak death in California and Oregon coastal forests and ramorum blight in European nurseries and landscapes (1), was detected in six Oregon nurseries in Jackson, Clackamas, and Washington counties from May to June 2003. The pathogen was isolated from: Viburnum bodnantense ‘Dawn’, V. plicatum var. tomentosum ‘Mariesii’, Pieris japonica × formosa ‘Forest Flame’, P. japonica ‘Variegata’ and ‘Flaming Silver’, P. floribunda × japonica ‘Brouwer's Beauty’, Camellia sasanqua ‘Bonanza’ and other cultivars, C. japonica, and Rhododendron × ‘Unique’. Samples of symptomatic tissues were plated on a Phytophthora-selective medium (PARP) and tested by polymerase chain reaction (PCR) (3). All samples positive for P. ramorum with PCR yielded P. ramorum isolates in culture. The isolates have the European genotype, mating type A1, except for the Camellia spp. isolates, which have the North American genotype, mating type A2 (2). Isolates are deposited in the American Type Culture Collection. Koch's postulates for this pathogen have been completed on V. bodnantense and C. japonica (1). To confirm pathogenicity on the new hosts, isolates from V. plicatum var. tomentosum ‘Mariesii’, Pieris × ‘Forest Flame’, Pieris × ‘Brouwer's Beauty’, and P. japonica ‘Variegata’ and ‘Flaming Silver’ were used to inoculate healthy plants of the same cultivars. For isolates from Rhododendron × ‘Unique’ and C. sasanqua ‘Bonanza’, pathogenicity was tested on Rhododendron × ‘Nova Zembla’ and C. sasanqua ‘Sutsugekka’ and ‘Kanjiro’. Three to five plants of each cultivar were inoculated and three to five were noninoculated. Zoospore inoculum was prepared on dilute V8 agar for one isolate from each host. Foliage of plants growing in 10-cm pots was dipped for 5 sec in a zoospore suspension (3 × 104 zoospores per ml) or sprayed to runoff with a hand mister (6 × 104 zoospores per ml). Control plants were dipped in or sprayed with sterile water. C. sasanqua plants were also inoculated by placing 6-mm mycelial plugs on individual leaves that had been wounded by piercing with a pin. Control leaves were wounded but not inoculated. Foliage was enclosed in plastic bags to retain humidity and the pathogen, and plants were incubated in a locked growth chamber (21 to 23°C). After 21 days, plants were examined for symptoms, and isolations onto PARP were made. All inoculated plants showed foliar symptoms, and P. ramorum was consistently isolated from inoculated plants, but not from asymptomatic control plants. On Rhododendron × ‘Nova Zembla’, nearly all leaves were wilted and dead, as were terminal buds and stems. Pieris spp. cultivars exhibited leaf and stem necrosis and defoliation. On V. plicatum var. tomentosum ‘Mariesii’, necrotic leaf lesions and defoliation of the lower leaves were observed. On C. sasanqua, necrotic lesions developed only on wounded leaves inoculated with mycelial plugs; these leaves abscised. Our results confirm the pathogenicity of Oregon nursery isolates of P. ramorum on V. plicatum var. tomentosum ‘Mariesii’, P. japonica × formosa ‘Forest Flame’, P. japonica ‘Variegata’ and ‘Flaming Silver’, P. floribunda × japonica ‘Brouwer's Beauty’, C. sasanqua and Rhododendron and complete Koch's postulates for several new hosts.

References: (1) J. M. Davidson et al. Online publication. doi:10.1094/PHP-2003-0707-01-DG. Plant Health Progress, 2003. (2) E. M. Hansen et al. Plant Dis. 87:1267, 2003. (3) L. M. Winton and E. M. Hansen. For. Pathol. 31:275, 2001.