J. L. Parke1, R. G. Linderman2, and E. M. Hansen3
Nursery and greenhouse crops together comprise Oregon's largest agricultural commodity. P. ramorum poses a threat to nursery production and export, but the magnitude of the threat is difficult to assess because the susceptibility of most horticultural plant species to this new pathogen is unknown. Quarantine restrictions prevent large-scale greenhouse inoculations with P. ramorum, so we developed a detached leaf assay to determine the potential susceptibility of nursery plants to Phytophthora ramorum foliar necrosis.
Freshly collected leaves are dipped for 5 sec, midway, petiole first, into a suspension of 6 x 104 zoospores mL-1. The inoculum is a combination of three Oregon isolates originating from rhododendron, tanoak, and evergreen huckleberry. Control leaves are dipped in sterile water. Leaves are placed in a moist chamber at 20°C for 7 days. Leaves are photographed digitally and the percent necrotic area quantified with image analysis software. A known susceptible host (evergreen huckleberry or rhododendron cv. 'Cunningham's White') is included in each assay as a positive control. Previous work (Parke, Linderman, and Hansen, 2002) demonstrated that symptoms that developed in the detached leaf assay reflect those resulting from inoculation of whole plants.
We have used this method to evaluate the relative susceptibility of more than 60 nursery plant taxa belonging to 24 families. Results indicate that a wide range of nursery plants is potentially susceptible to P. ramorum foliar necrosis. Some of the most susceptible plants tested to date include Acer palmatum (Aceraceae), Viburnum tinus (Caprifoliaceae), Oxydendrum arboreum (Ericaceae), Rhododendron 'Exbury Hybrids' (Ericaceae), Ribes sanguineum (Grossulariaceae), Gleditsia triacanthos (Fabaceae), Buddleia davidii (Loganiaceae), Clematis montana (Ranunculaceae), Ceanothus impressus (Rhamnaceae), and Hamamelis vemalis (Styracaceae). No symptoms were observed on inoculated Cornus florida (Cornaceae), Hypericum 'Hidcote' (Hypericaceae), Chaenomeles speciosa (Rosaceae), Cotoneaster multiflorus (Rosaceae), Photinia serrulata (Rosaceae), Rhaphiolepis umbellata (Rosaceae), or Philadelphus coronarius (Saxifragaceae). Considerable variation was observed among different species within the same genus (Acer, Rhododendron, Viburnum), or even different cultivars within a single species (Acer palmatum). Our results indicate that many nursery and landscape plants should be surveyed for early detection of P. ramorum infection.
The detached leaf assay appears to provide a simple, reproducible method to determine potential susceptibility of plant hosts to P. ramorum. Inoculum can be adjusted to a consistent, quantifiable dose. Incubation conditions can be maintained at a constant environment and containment is relatively simple. Small quantities of plant leaves are used, and many plant species may be assayed simultaneously.
