J. L. Parke1, E. M. Hansen2, and R. G. Linderman3
Although Phytophthora ramorum can infect a broad range of plant species, some hosts appear to contribute to the disease epidemic more than others. One reason for this is that hosts could differ in their capacity to support inoculum production by the pathogen. We assessed the sporulation potential of P. ramorum on various host species under controlled conditions by quantifying zoospores and chlamydospores produced from inoculated leaf disks during a 7-day period. Host species included in the assay were California bay laurel (Umbellularia californica), tanoak (Lithocarpus densiflorus), madrone (Arbutus menziesii), Pacific rhododendron (Rhododendron macrophyllum), and evergreen huckleberry (Vaccinium ovatum). Additional non-host plant species included in the assays were vine maple (Acer circinatum), salal (Gaultheria shallon), red alder (Alnus rubra) and Oregon white oak (Quercus garryanna).
Leaf disks (6 mm diam) from each plant species were dipped in a zoospore suspension (6 x 104 zoospores mL-1) and incubated on moist filter paper in a petri dish for 24 hr. Each disk was then floated on the surface of sterile water (1 mL) placed in a well of a 24-well microtitre plate. Control treatments consisted of leaf disks dipped in sterile water rather than in the zoospore suspension. In each assay, there were 5 replicate leaf disks per plant species and inoculation treatment, and treatments were completely randomized within the microtitre plates.
Leaf disks and the well contents were examined daily for sporangia, zoospores, and chlamydospores. If sporangia were observed, zoospore release was induced by chilling (at 4° C) the microtitre plates containing the leaf disks, then returning the plates to room temperature. After removal of the disks, the contents of each well were mixed, and 25-100 _L of the suspension was plated onto modified PARP medium to enumerate colonies arising from zoospores. Chlamydospores surrounding the leaf disks were counted 7 days after inoculation. Leaf disks were then cleared in 10% KOH, rinsed in water, acidified in dilute HCl, and stained in 0.05% trypan blue in lactoglycerin to reveal additional chlamydospores in or on the plant tissue.
The capacity for P. ramorum to produce sporangia, zoospores, and chlamydospores on leaf disks was greatest and most rapid on California bay laurel. Abundant sporangia and active zoospores were released within 24 hr following placement of leaf disks in the wells, and zoospores continued to be released for several days. Tanoak also supported numerous sporangia and zoospores soon after infection. On madrone leaf disks, only chlamydospores were produced. On other hosts, a few to many sporangia were produced over the course of several days, with chlamydospore production following. Of the species not reported as hosts, inoculum production was most abundant on vine maple and salal.
Despite the artificial nature of this assay, results indicate that certain foliar hosts enable rapid and prolific reproduction of the pathogen, whereas other hosts do not. The possible epidemiological ramifications of this finding will be discussed.
