E. Hansen1, W. Sutton2, J. Parke2, and R. Linderman3
Phytophthora ramorum causes girdling cankers on tanoak (Lithocarpus densiflorus) and foliar and dieback symptoms on a few other hosts, including native rhododendron (Rhododendron macrophyllum) and evergreen huckleberry (Vaccinium ovatum) in forests of SW Oregon. Our objective was to test the potential susceptibility of other forest trees native to Oregon.
We inoculated freshly cut logs, 10-20 cm dia. and about 1 m long, small diameter (<1cm) stems of potted plants, and leaves of native conifer and angiosperm tree species. For log inoculations, plugs of bark (5 mm dia.) were removed to the cambium, and a matching plug of colonized agar (or sterile agar) was inserted; wounds were covered by wet gauze and foil. Bark was stripped after 5-7 weeks and extent of phloem necrosis was measured. Isolations were made from canker margins. Stem inoculations were made by inserting a small piece of colonized agar into a horizontal slit cut to the cambium. Detached leaves and intact plants were also dipped in zoospore suspension without any overt wounding.
Tree species differed in the extent and pattern of symptom development following the different inoculation treatments. Phloem necrosis following log inoculation developed in most tree species tested, but large, expanding lesions were regularly seen only in a few members of the Fagaceae. A broader range of species was susceptible to stem inoculations, but girdling cankers formed on only about 18 of 41 species tested, including several conifers. Leaf dip resulted in variable symptoms on all hosts except willows and poplars, and conifers. Dipping intact shoot tips of whole plants in zoospores gave perhaps the most realistic test. Foliar symptoms were similar to the leaf dip test, but disease progressed into the twigs of only a few species of Ericaceae, Fagaceae, and several conifers.
The combined inoculation results and field observations suggest that P. ramorum causes three distinct disease syndromes on different plant species. "Sudden Oak Death," characterized by lethal bole cankers, is presently confined to a few species of the family Fagaceae. "ramorum leaf blight" follows exposure of foliage of many (most?) angiosperm plant species to sufficient inoculum levels. "ramorum shoot dieback" results from foliar infection and/or direct infection of stems on a limited number of species, including tanoak, madrone, some Vaccinium and Rhododendron species, and several conifers.
