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New Publications

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Hansen, E.M. 2015. Phytophthora Species Emerging as Pathogens of Forest Trees. Current Forestry Reports. DOI: 10.1007/s40725-015-0007-7.

Species of Phytophthora are prominent in lists of emerging threats to forest ecosystems. We explore the conditions leading to and the consequences of the emergence of some Phytophthora species from their presumptive coevolved roles in undisturbed forest ecosystems to destructive agents as invasive forest pathogens. Phytophthora species are widespread, relatively abundant, very diverse, and poorly understood in many relatively undisturbed forest ecosystems. Three examples are examined in detail to illustrate the range of pathways to emergence and the varied consequences to forest environments. Phytophthora lateralis causes Port-Orford cedar root disease in western North America and now Europe. Phytophthora ramorum is causing unprecedented mortality in oak and tanoak forests in California, as the cause of sudden oak death, and is killing planted larch in the UK, and Phytophthora cinnamomi kills trees in parts of the world where it has been introduced. Active programs are underway in each case to manage, if not eliminate, their damage. In no case, however, has eradication been achieved. Prevention, by blocking initial introduction, has the highest probability of success.

Roy, B.A.; Alexander, H.M.; Davidson, J.; Campbell, F.T.; Burdon, J.J.; Sniezko, R.; and Brasier, C. 2014. Increasing Forest Loss Worldwide from Invasive Pests Requires New Trade Regulations. Frontiers in Ecology and the Environment. 12(8): 457–465.

Loss of forests due to non-native invasive pests (including insects, nematodes, and pathogens) is a global phenomenon with profound population, community, ecosystem, and economic impacts. We review the magnitude of pest-associated forest loss worldwide and discuss the major ecological and evolutionary causes and consequences of these invasions. After compiling and analyzing a dataset of pest invasions from 21 countries, we show that the number of forest pest invasions recorded for a given country has a significant positive relationship with trade (as indicated by gross domestic product) and is not associated with the amount of forested land within that country. We recommend revisions to existing international protocols for preventing pest entry and proliferation, including prohibiting shipments of non-essential plants and plant products unless quarantined. Because invasions often originate from taxa that are scientifically described only after their introduction, current phytosanitary regulations – which target specific, already named organisms – are ineffective.

Stream monitoring will be expanded in the Northern Olympic Peninsula, WA for Phytophthora ramorum

gary.maguire

Northern Olympic Peninsula, WA Phytophthora ramorum stream monitoring will be expanded in spring 2015 in an effort to identify the source of inoculum contaminating the Dungeness River near Sequim, WA.  The river was found positive twice in 2013 and is not in the vicinity of a P. ramorum-positive nursery.  To date, follow-up sampling of streams in the area have not yielded information about the inoculum source.  The Chastagner lab at Washington State University, Puyallup will oversee the additional monitoring through a volunteer program and increase efforts to genotype isolates and DNA samples from waterways to help clarify the inoculum origin.  Several stormwater retention ponds in Pierce County will also be monitored to determine if landscaped areas in new developments may harbor the pathogen.

2014 National P. ramorum Early Detection Survey of Forests Summary

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2014 National P. ramorum Early Detection Survey of Forests Summary – Along the West Coast, California, Oregon, and Washington conducted stream surveys using bottle of bait (BOB) and/or leaf baiting.  In CA, 146 sites were sampled over 5 baiting periods, with 19 positive samples collected (13 from previously positive locations).  Six of the positives were from watersheds that had not previously tested positive (See the January COMTF report for more information.).  In OR, 11 sites were sampled over 19 baiting periods, resulting in the detection of 6 positive waterways.  The OR survey samples were tested via culturing and PCR, with PCR diagnostics identifying 10 samples that were not found through culturing.  In WA, both BOB and leaf mesh bag sampling were used.  The two positive samples found were with the bait bag sampling method (not BOB) and from the same stream.

Four of the 9 participating eastern states had 9 P. ramorum-positive stream samples – AL (4), FL (1), MS (2), and NC (2).  Two of the positive streams (AL-1; FL-1) have not been previously positive and are each associated with a positive nursery.  Another Alabama positive stream has not been previously positive but is not associated with a positive nursery.  Plans for vegetation sampling near this stream are underway.

Bottle of bait (BOB – whole leaves and leaf pieces together in a bottle of collected water) monitoring protocols were adopted to detect P. ramorum for the forest stream survey in 2014.  BOB whole leaves detected five positive streams and leaf pieces detected four. Nine environmental plant samples from Georgia were tested for P. ramorum, but all were negative.  As found in previous years (2010-2013), the spring (March – May) bait period resulted in more positive samples (7) than the late season (October – November; 2). In total, 665 BOB samples were processed from 85 different sites in 9 eastern states (AL, FL, GA, MS, NC, NY, PA, SC, and TX).

BMP for Native Plant Growers

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Best Management Practices for Native Plant Nurseries

CansMany of the plants that are grown in the PNW forest and native plant nurseries are hosts for P. ramorum. In addition to the economic impact that nurseries would suffer if P. ramorum was to be detected, the movement of infected plant material from these nurseries would likely result in the introduction of the pathogen into forest or natural sites. This may have significant economic and/or ecological impacts.

There are a number of phytosanitary measures that native plant nurseries can take to minimize the risk of inadvertently introducing P. ramorum or other Phytophthoras into a nursery site. Given the continued spread of P. ramorum inoculum from ornamental nurseries into nearby waterways, one of the keys is to treat any water that is used from streams or lakes for irrigation.

Another key is to be very careful about the plant material that is brought into the nursery and inspect new shipments closely for symptoms, especially if they are coming from an area where P. ramorum is established.

Native plants used in restoration sites are at risk for spreading P. ramorum into forests if they are infected. It is important to be aware of the source of irrigation water for these plantings.

Phytophthora and other diseases can spread in residual soil and plant material in pots that are re-used. Pots can be treated with disinfectants or heat to kill these organisms.

California Society for Ecological Restoration Quarterly Newsletter Summer Volume 26, Issue 2 has these two articles:

“Nursery Plants as a Pathway for Plant Pathogen Invasion” by Susan J. Frankel, Kathy Kosta, and Karen Suslow

“Solarization: A Simple and Low-Cost Method for Disinfesting Horticultural Containers” by Karen Suslow and Kathy Kosta

A Phytophthora tentaculata Pest Alert is now available. P. tentaculata is an emerging pathogen in California native plant nurseries and restoration plantings.

So far, P. ramorum has not escaped into the natural environment in the PNW except for in streams associated with positive nurseries and landscapes. Research on the amount of inoculum needed in water for infection, susceptibility and sporulation potential of plant hosts, and other topics will help us determine the level of risk to our forests. In the meantime, it pays to be cautious when working with host plants.

Photos of native plant nursery best (and worst) management practices

If you have ideas or photos to add to these pages, let Marianne know.

Thanks to Regina Johnson for many of the photos used in this section.

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

WA native plants

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Pacific Northwest Plant Hosts

 026 smA 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.

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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