Rhododendrons are an important nursery crop in the Pacific Northwest and have accounted for about 78% of the infected plants associated with positive nurseries in Washington since 2004. In an effort to identify the risk that certain Rhododendron species and hybrids pose in spreading this pathogen we have been examining the susceptibility and sporulation potential of P. ramorum on 42 Rhododendron species and 58 hybrids using an isolate of P. ramorum originally taken from Rhododendron. This isolate belongs to the NA1 lineage, which is the most commonly detected lineage of P. ramorum in Washington nurseries. Detached leaves were inoculated with a zoospore suspension on the upper and lower surfaces in both wounded and non-wounded treatments. Lesion area was measured after 5 and 10 days and production of sporangia and chlamydospores was assessed for each species and hybrid.

There was a great deal of variability among Rhododendron species and hybrids in their susceptibility to infection by P. ramorum, severity of symptoms, and production of sporangia and chlamydospores. Several species and hybrids could be considered resistant, with low incidence and severity of disease and low levels of sporulation. These would be good candidates for nursery production and possibly for future hybridizing material. Likewise, several species and hybrids that were very susceptible and supported high levels of sporulation were identified. In most cases lesions were larger on wounded than non-wounded leaves, and there was a higher infection frequency on the lower leaf surface than on the upper surface.

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Contact: Gary Chastagner, 253-445-4528 | WSU Puyallup Research & Extension Center, 2606 West Pioneer, Puyallup, WA, 98371-4998 USA
Last updated January 2, 2013

Western Washington is considered to be at high-risk for the spread of P. ramorum. Limited information is available regarding the sporulation of P. ramorum on hosts in western Washington forests. Research is underway to determine the potential for P. ramorum to sporulate on known and potential hosts that commonly occur in western Washington forests. This information will increase our understanding of potentially important epidemiological hosts and provide insights on the role they may play in the spread of P. ramorum in forest ecosystems where other important epidemiological hosts, such as California bay laurel and tanoak do not occur.

Studies with known conifer hosts, such as Douglas-fir, indicate that infection occurs during the early stages of shoot emergence. Thus, sporulation on hosts such as madrone and big leaf maple which potentially have foliage within the mid to upper canopy of the forest probably pose the greatest threat to spreading the disease to established conifers. Although one small preliminary study suggests that P. ramorum does not produce sporangia on infected Douglas-fir and grand fir shoots, additional tests are needed to confirm this and determine the potential for it to sporulate on other conifer foliage within Washington’s forests. Although potentially less important as a source of inoculum for standing trees, sporulation on understory shrubs, like Pacific rhododendron and salal, may be important sources of inoculum that may contaminate logs during harvest.

A laboratory study using detached leaves of some common broadleaf hosts found in the PNW was undertaken in summer of 2009 and 2010. Leaves were inoculated with a zoospore suspension of an NA1 isolate of P. ramorum and lesion area and infection frequency was evaluated. If P. ramorum was recovered from a leaf that did not show a visible lesion, it was considered to be infected asymptomatically. Sporulation potential of foliage of each plant species was also determined.

In general, the results of this study indicate that western Washington forests are not at high risk for damage caused by P. ramorum, based on the host plants tested. However, this is a subset of the many plant species that occur, and there may be a host species that is either extremely susceptible to infection or a prolific sporulator, that was not tested in this study. Plants posing the smallest risk of P. ramorum establishment were generally invasives and/or riparian species. The highest risk plants were commonly found in forested environments. These were fairly susceptible to infection and produced more chlamydospores than sporangia in their foliage.

Chlamydospore production was higher than sporangia production on many hosts in western Washington forests that were examined in this study. In other systems, such as bay laurel (Umbellularia californica) in California, and Rhododendron ponticum in the UK, P. ramorum outbreaks are driven by high concentrations of sporangia produced on foliage of these hosts. None of the Washington hosts tested produced as many sporangia as U. californica. Chlamydospores are a means by which P. ramorum can persist on a site in soil and decaying foliage, but will probably not produce large amounts of inoculum unless they germinate directly into sporangia, which can occur in flooded soils.

Publications

Phytophthora ramorum isolated from California bay laurel inflorescences and mistletoe: possible implications relating to disease spread 174 KB
Gary A. Chastagner, Kathy Riley, and Norm Dart

Contact: Gary Chastagner, 253-445-4528 | WSU Puyallup Research & Extension Center, 2606 West Pioneer, Puyallup, WA, 98371-4998 USA
Last updated January 2, 2013