S.A. Tjosvold1, S.T. Koike2, J.M. Davidson3, and D.M. Rizzo3
Phytophthora ramorum, the causal agent of the disease commonly known as Sudden Oak Death is a prevalent pathogen in California with its effects evident in 12 counties and found on 14 different oak, tanoak and non-oak hosts. Rhododendron and azalea (Rhododendron) are very important horticultural commodities and potentially could be important long-distance carriers of Phytophthora ramorum through commercial trade. Although plants of the rhododendron subgenus has been found naturally infected in commercial nurseries and landscapes, plants in the azalea subgenus, so far, have not been found naturally infected. The question arises as to whether azaleas are susceptible to P. ramorum, and, if so, to what extent.
Twenty commercially available cultivars or species were tested for susceptibility to P. ramorum in a series of experiments. Plants were selected to include a broad genetic diversity, including deciduous and evergreen selections. Rhododendron 'Cunningham's White' was used in all experiments as a standard susceptible rhododendron cultivar. Susceptibility was gauged using several inoculation procedures.
P. ramorum inoculations were made on leaves on potted plants in a greenhouse (18 o C mean) or on detached leaves and incubated in a dark chamber (20 o C) at high humidity. Inoculum included: 1) mycelial agar plugs with pin wounding or 2) mycelial agar plugs without pin wounding, or 3) zoospore suspension.
With inoculum plugs, pin wounding proved necessary to reliably infect intact plants in the greenhouse or detached leaves in the incubation chamber. With zoospore inoculum, infection was most successful when leaves were removed, inoculated, and incubated in the humid chamber rather than the leaves inoculated and incubated, intact, on the plant in the greenhouse.
Zoospore inoculation of detached leaves resulted in small lesions forming on all cultivars. P. ramorum was successfully isolated from these lesions in all but one cultivar. In those leaves where lesions were formed, the pathogen was isolated in 17 to 100 percent of those cases.
The deciduous azaleas 'Northern Hilights' and R. occidentale, and the standard susceptible rhododendron, 'Cunningham's White', proved to be highly susceptible. Deciduous azaleas generally were more susceptible than evergreen azaleas.
