Phytophthora alni subsp. uniformis (PAU) was isolated from soil beneath alder from two riparian areas in 2007; one area was in south-central and one in interior Alaska. This is the first time this hybrid pathogen has been found in North America. Alder Phytophthora, primarily subsp. alni (PAA), is a well documented lethal root and collar disease of alder in nearly a dozen European countries. The PAU subspecies, however, is considered less aggressive than PAA, though our understanding of this is evolving. Finding PAU in two remote, unmanaged locations in Alaska is surprising and perplexing; the threat to Alaskan alder from this pathogen is unknown. No root or root collar symptoms of this pathogen have been noted in Alaska. Perhaps PAU has co-existed benignly in Alaska with alder and has not been noted due to the lack of surveys such as those conducted in 2007. Monitoring and research related to this pathogen is continuing in 2008.
see the full briefing...
The US Forest Service, APHIS and Alaska Division of Agriculture compiled a brief one page synopsis for PAU. Please see this "fact sheet" for a basic outline of current knowledge. This synopsis also addresses questions concerning 1) recognizing the pathogen, 2) risk for alder in Alaska, and 3) what the next steps should be.
On February 29, the APHIS’ New Pest Advisory Group (NPAG) issued the following official
recommendations for Phytophthora alni, alder root rot:
- The NPAG recommends no change in the PPQ policy regarding Phytophthora alni, retain as
reportable/actionable, and list P. alni as such in the Pest ID database. Action Leader: Joe Cavey,
PPQNIS.
- The NPAG recommends that PPQ establish a list of Phytophthora of concern to the United
States and require offshore testing for these species with negative results documented on a
phytosanitary certificate prior to any susceptible host entering the United States. CPHST will
develop this list in collaboration with Matt Royer (EDP), Bill Alley (PRI), and others. When the
new Q37 regulation is finalized, hosts of Phytophthora of concern to the United States will be
placed on the Not Allowed Pending Risk Assessment (NAPRA) list.
Action Leader: Kim Schwartzburg, PPQCPHST.
- The NPAG recommends adding Phytophthora alni to a list of Phytophthora of concern to the
United States. Action Leader: Kim Schwartzburg, PPQCPHST.
- The NPAG recommends incorporating Phytophthora alni into a New Pest Response
Guideline that will address Phytophthora of concern to the United States. Action Leader: Russ
Bulluck, PPQEDP.
Dr. Gerry Adams, Michigan State University
At thirty streams in Alaska, Phytophthora species were baited and trapped from roots, soils, and water sources using Rhododendron leaves.
Baits with Rhododendron leaves.
- Following a 48 hour incubation, the baits were removed from contact with the plants, soils, and water. They were washed and incubated at room temperature to 17 C, depending on resources.
- The soil was placed in styrofoam bowls and saturated. Three rhododendron leaves were placed on each and incubated up to 7 days.
- As lesions formed over 7-days, isolations from the margins of lesions were transferred to Phytophthora selective medium.
- From these media, isolates were transferred to V-8 juice medium containing ß-sitosterol to stimulate formation of morphologically informative sexual spores (oospores).
- Later, plugs from these plates were floated in soil-extract to stimulate formation of a second type of morphologically informative spore producing organelle (sporangia).
- Permanent slide mounts will be prepared of the oospores and sporangia and used in microscopic studies and vouchers in conventional identification of the Phytophthora species.
- Additionally, species that are intractable to morphological identification such as P. gonapodyides will be identified based in DNA sequence homology to identified reference strains in NBCI Genbank data repository.
Panguigue Creek, one of the two positive Phytophthora alni subsp. uniformis sites in Alaska.
