Month: November 2023

SOD – 3 strains spreading in the West …

locations of P. ramorum in forests of Oregon in 2023

In a recent blog I offered several critiques of APHIS’ new Phytophthora ramorum risk assessment regarding possible establishment of the causal agent of sudden oak death, in the eastern U.S. states. One of my objections was the brevity of its discussion of the likelihood of sexual combination of the recently introduced EU1 strain with the strain established in North America, NA1 and – more recently – NA2.

This blog provides updates on the status of the Phytophthora ramorum invasion in California and Oregon. My information comes primarily from the newsletter posted by the California Oak Mortality Task Force (COMTF),  supplemented by presentations at the recent on-line meeting.  

Research by several scientists, including Tyler Bourret, now with USDA Agricultural Research Service, [summarized in the November 2023 COMTF annual meeting] reported that 216 species are now recognized in the genus Phytophthora.

Establishment of Additional Strains of the Pathogen

Scientists now recognize 12 strains of P. ramorum (Sondreli et al., summarized in COMTF newsletter for August 2023). Three of these strains are established in western North American forests. All three – NA1, NA2, & EU1 – are established in southern Curry County, Oregon. Two of the three – EU1 & NA1– are established in neighboring Del Norte County, California. The genetic lineage of the EU1 population in Del Norte points to a link to the Oregon outbreak.  [Robinson/Valachovic presentation to COMTF annual meeting November 2023] Given the poor record of efforts to prevent additional introductions of P. ramorum to the United States (the APHIS risk assessment notes that the pathogen has been introduced eight to14 times – or more! — in California), continued introductions of strains not yet established in the U.S. appear likely. Once a strain is established in a North American nursery, it is very likely to spread to nurseries – and possibly forests – in other parts of the country. Remember, the risk assessment reported that P. ramorum has probably been moved over a thousand times on nursery stock from West Coast nurseries across the U.S.

P. ramorum-infected Rhododendron; photo by Jennifer Parke, Oregon State University

Why this matters

Phytophthora ramorum can reproduce sexually only when gametes of the two different mating types (A1 & A2) combine. Most of the North American populations are A2 mating type and most European populations are A1. Establishment of the European EU1 in Oregon and California increases the likelihood that sexual reproduction will occur, which in turn increases the probability that the pathogen will evolve. Sexual combination between NA2 (mating type A2) & EU1 (mating type A1) has occurred at least once – in a nursery in British Columbia. Authorities believe this hybrid has been eradicated. However, the possibility of such matings remains.

The most widespread strain in North America is NA1. It was first detected in the forests north of San Francisco in the middle 1990s; and in Oregon in 2001. Infestations of NA1 are now found from central Curry County, Oregon to Monterey County, California.

The EU1 lineage was first detected in Oregon in 2015. How did it get there since it was previously known only in Europe? The outbreak in Del Norte County, California – detected in 2020 – apparently is associated with the Oregon infection. [Robinson/Valachovic presentation to COMTF annual meeting November 2023] Both states attempted eradication, but the strain is well established. By 2023, the Oregon infestation was detected spreading at sites where intensive surveys in previous years detected no symptomatic trees. In California, new centers of infection have been detected along additional tributary creeks in the area. Scientists expect these infections to spread downhill. Control efforts and even surveys have been hampered by a large fire in the area, which diverted needed personnel and funding. [COMTF newsletter for October 2023 & Robinson/Valachovic]

The NA2 lineage has been found in some nurseries in the Pacific Northwest since 2005. The first detection in forests occurred near Port Orford, Oregon in 2021. Port Orford is 30 miles north of Gold Beach – the hitherto northern extent of the SOD infestation. Oregon authorities believed this signaled a new introduction to the state. By 2023, three sites in the state are now infested with this strain. [Ritokova presentation to COMTF annual meeting November 2023] Oregon now focuses its control efforts on NA2 outbreaks near Port Orford.

In California’s Del Norte County, there are now infestations of two strains of opposite mating types ~ 6 miles apart.The forests between them are conducive to infection, so interactions are likely. Robinson & Valahovic [COMTF annual meeting November 2023] ask how land managers should deal with any interactions. I ask – given the likelihood of hybrids forming – shouldn’t the APHIS risk assessment have tried harder to analyze this risk to the East?

Meanwhile, the NA1 strain continues to spread

In Oregon, the NA1 strain has spread 18 miles to the north and eight miles to the east since 2001 [Ritakova COMTF newsletter October 2023]. In California, spread after the wet winter of 2022-2023 has so far been less than expected. The SOD Blitz [Garbelotto at COMTF annual meeting November 2023] found that the statewide rate of positive trees rose from 7.1% in 2022 to 8.8% in 2023. In the Big Sur region some canyons now test negative that once were positive. Scientists think the negative tests reflect the multi-year drought. Scientists expect the spread will be more visible next year – especially if there is a second wet winter.

As noted above, the exception is in Del Norte County – an area described by CAL FIRE forester Chris Lee as a very wet “pathology” site. SOD (NA1 strain) was first detected in the area north of Crescent City in 2019 [Robinson and Valachovic]. This outbreak could not be re-confirmed for three years, despite intensive surveys. But, in 2022, scientists detected a new concentration of dying tanoak. The infected area is near both rare plants associated with serpentine soils and Jedediah Smith State Park, a unit of Redwood National Park. [Robinson] Meanwhile, the infestation of EU1 strain was first detected in 2020; it has expanded in 2022 and 2023.

In addition to spread facilitated by weather, we also see a continuing role in pathogen transfer via movement of shrubs intended for planting. In fall 2022 Oregon authorities were alerted by a homeowner to an outbreak in Lincoln City, Oregon. This was alarming for four reasons:

  • it was 201 miles north of the generally infested area in southern in Curry County.
  • it was well established and had apparently been present for many years.
  • P. ramorum was not detected in any associated waterways, raising questions about the efficacy of this standard detection method for use in community detections.
  • one of the infected plants was a new host: western sword fern (Polystichum munitum).

Fortunately, the infection has not (yet) been detected in nearby natural forests. Perhaps this is because there are no tanoaks this far north.

Detection Difficulties

Forest pathologists report several examples of outbreaks involving dozens of trees or plants suddenly being detected in areas which had been surveyed intensively in preceding years with no detections. See Robinson/Valachovic presentation [COMTF annual meeting November 2023, re: both EU1 & NA1 strains in Del Norte County]. I noted above that streams near the Lincoln City, Oregon neighborhood outbreak did not test positive. Nor did water associated with a positive nursery in Oregon[description of Oregon Department of Agriculture nursery regulatory program in COMTF newsletter for August 2023]. Stream baiting is an important component of detection surveys, so I worry about the possible implications of these negative results.

Identification of Additional Hosts  [all from COMTF newsletter for August 2023.]

  • silverleaf cotoneaster Cotoneaster pannosus (an invasive non-native plant species) 
  • “Mountain Moon” dogwood Cornus capitata [host previously identified in the United Kingdom]
  • western swordfern (Polystichum munitum) (discussed above)
Oregon P. ramorum eradication attempt; photo by Oregon Department of Forestry

Management

Oregon has tried to manage SOD in the forest since its first detection, but the pathogen’s spread and the recent appearance of two additional strains have overwhelmed the program. One hope was to find a less expensive eradication or containment method. For 20 years, attempts to suppress the disease has focused on eradicating local populations of tanoaks (Notholithocarpus densiflorus) because they are the principal host supporting sporulation in Oregon. When an outbreak has been detected and delimited, they first kill the tanoaks with herbicides to prevent resprouting from the roots. The trees are then felled, piled, and burned. This treatment costs $3,000 – $5,000 / acre. Scientists tested whether they could greatly reduce the cost of the suppression programs by leaving tree boles standing after they have been killed by herbicide. Unfortunately, leaving dead, herbicide-killed trees standing increased sporulation, so this approach would probably exacerbate pathogen spread. [See Jared LeBoldus presentation to COMTF annual meeting November 2023]

Worrying Developments in Europe

In Ireland, sudden larch death – caused by the EU1 strain on Japanese larch (Larix kaempferi) – has spread to several counties. This strain is also causing disease on European beech (Fagus sylvatica) & Noble fir(Abies procera) in locations where these tree grow in association with nearby heavily infected Japanese larch. The EU2 lineage was found in late 2021, infecting L. kaempferi at one site.

Several other Phytophthora species are causing disease on trees, including P. lateralis on Lawson’s cypress, Port-Orford cedar (Chamaecyparis lawsoniana); P. pseudosyringae on Japanese larch; and P. austrocedri on trees in the Juniperus and Cupressus genera.

[information about Ireland from R. O’Hanlon, summarized in COMTF newsletter for August 2023]

Regulation

The European Union has relaxed phytosanitary regulation of Phytophthora ramorum. Previously the species – all strains – was considered a quarantine pest. Now its regulatory status depends on the origin of the infected material. “Non-EU isolates” of Phytopththora ramorum are still quarantine pests (presumably the two North American strains [NA1 & NA2] and the eight other strains identified in Asian forests). These pests are treated as the most serious pests in the Union; when they are detected, extensive control actions must be taken. “EU isolates” (presumably EU1 & EU2) are now treated as regulated non-quarantine pests. The focus is to limit the spread of these on plants for planting only.

The European Union and USDA APHIS regulatory emphases differ to some extent (APHIS does not regulate P. ramorum in natural settings, only interstate movement via, inter alia, the nursery trade). However, I am worried that both seem intent on minimizing their regulatory programs.

Arbutus canariensis; photo by Moreno José Antonio via Plantnet

Another region at risk

Macaronesia is a group of several North Atlantic islands,e.g., Madeira and the Azores, Canary, and Cape Verde islands. The islands have climates similar to areas affected by P. ramorum. The Macaronesian laurel forest is a remnant subtropical evergreen forest which shares some plant taxa with those that host the pathogen elsewhere. Moralejo et al. found that, overall, plant species showed considerable tolerance of the pathogen. However, P. ramorum was “rather aggressive” on Viburnum tinus, Arbutus canariensis and Ilex canariensis. Furthermore, mean sporangia production on five Macaronesian laurel forest species was similar to levels on Umbellularia californica, a key host driving the SOD epidemics in California.Moralejo et al. concluded that there is a moderate to high risk of establishment if Phytophthora ramorum were introduced in the Macaronesian laurel forest. [Study summarized in October 2023 COMTF newsletter.]

Important Research

The COMTF August newsletter reports exciting work developing improved detection tools for Phytophthora species, especially P. ramorum. Sondreli, Tabima, & LeBoldus have developed a method to quickly distinguish among the four most common clonal lineages (NA1, NA2, EU1 and EU2). These assays are sensitive to weak concentrations and effective in testing a variety of sample types including plant tissue and cultures. Oregon State University is already using in its diagnostic laboratory.

YuFang, Xia, Dai, Liu, Shamoun, and Wu have developed a simple, rapid, sensitive detection system for the molecular identification of P. ramorum that does not require technical expertise or expensive ancillary equipment. It can be used in laboratory or using samples collected from the field.

Quiroga et al. found that thinning – with or without burning of the slash – significantly reduced stand density and increased average tree size without significantly decreasing total basal area. This effect persisted for five years after treatments – especially when supported by follow-up basal sprout removal. Preventative treatments also significantly increased dominance of tree species not susceptible to Phytophthora ramorum.

In a study summarized in the October 2023 COMTF newsletter, Bourret et al. reported results of nearly 20 years of leaf baiting in watersheds covering an 800-mile section of the Pacific Coast in northern and central California. They found 22 Phytophthora & Nothophytophthora species. Several – including P. ramorum — were abundant and widespread. Some isolates in northern California differ from those found elsewhere. Mitochondrial sequences revealed multiple hybridization events between P. lacustris and P. riparia.

Bourret et al. also found that P. pluvialis is probably native to Western North America. The strain invasive on conifers in New Zealand probably originated in California rather than Oregon or Washington.

Jared LeBoldus and colleagues are studying the ecological impact of tanoak mortality in Oregon forests. [Summarized in November 2023 COMTF newsletter.] They expect impacts at various trophic levels and functions. Preliminary findings regarding the plant community show increases in understory and herbacious species diversity; a shift away from tanoak to Douglas-fir; and increased coarse woody debris. These findings are similar to results from studies in central California by Dave Rizzo and colleagues at UC Davis. LeBoldus is now studying the microbiome of plant leaves; soil mycorrhizal diversity; invertebrates and pollinators (loss of the large annual flower crop of tanoaks presumably affects pollinators). They hope in the future to study small mammal communities (which they expect to be affected by the loss of acorns).

Jared LeBoldus and colleagues also reported early results of genomic studies exploring disease resistance in tanoaks. Various scientists started such studies in the past, but so far all efforts have petered out due to absence of sustained funding, support from agency management, and links to facilities with the necessary tree improvement/breeding resources. (See Richard Sneizko’s description of requirements for resistance breeding, here.) I hope this project proves more sustainable.

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

or

www.fadingforests.org

Succession: “novel drivers” change the trajectory

hardwood regeneration in northern Virginia forest; photo F.T. Campbell

I have posted several blogs recently about tree species’ regeneration. One blog found poor regeneration of many species throughout forests of the eastern United States. Regeneration is particularly poor in the Great Lakes region, western New York and Pennsylvania, along the Mid-Atlantic and New England coasts, and the coastal plain from southern South Carolina to eastern Texas.

A second blog focused on forest succession in New Hampshire. These findings, by Ducey et al., explicitly recognized the impact of non-native tree-killing insects and pathogens. A third article (Payne and Peet, 2023; full citation at the end of this blog) reports similar findings in North Carolina – and explicitly says that the same conditions are found in forests across the eastern United States.

The locations of neither in-depth study – New Hampshire or North Carolina – include those identified by Potter and Riitters (2022) as suffering particularly poor regeneration.

Payne and Peet find that forest succession in the Piedmont region of North Carolina is not proceeding as expected, based on earlier studies conducted in the same region. The differences are apparent at both the canopy and understory levels. Especially notable is the low recruitment of oaks (Quercus species) and hickories (Carya species) – the genera which previous studies indicated would be the climax taxa. One explanation is the disappearance since early in the 20th Century of fire as a driver of disturbance.

The understory communities are also novel, due largely to invasive species: dramatic loss of flowering dogwood (Cornus florida) killed by the non-native pathogen dogwood anthracnose (Discula destructiva), plus overcrowding of the shrub level by invasive plant species. Other drivers are probably suppression of growth of woody species caused by excessive deer herbivory, and overall accelerated shifts in successional trajectory due to hurricane damage.

flowering dogwood autumn display; F.T. Campbell

Forests in eastern North America in the 21st Century face several drivers of change that are either novel or greatly heightened. In addition to the disappearance of chronic fire, these are frequency and timing of hurricanes, feeding by herbivore populations, and introduction of non-native tree-killing pests and plants. Payne and Peet say scientists and managers need to consider these additional drivers – and their interactions! – when anticipating successional change.

Like Ducey et al. in New Hampshire, Payne and Peet used 80 years of data from 33 permanent plots established and 55 years of data from another 3 plots. Twenty-eight of the plots are transitioning from loblolly pine (Pinus taeda) to hardwood dominance; eight plots have been mixed-age hardwood stands since before the study plots were established.

In the North Carolina piedmont, the composition of canopy trees in plots evolving from pine compared to hardwood stands continue to be different 90–120 years after succession began. Canopy trees in upland and bottomland hardwood stands also differ. These differences reflect the relative species in the forest at the initiation of succession dynamics. Hurricanes – especially Hurricane Fran in 1996 – apparently accelerated succession in some plots by toppling the oldest pines. Despite the persistent differences, the species compositions of both canopy and subcanopy layers are trending toward increasing similarity.

deer-damaged red maple; photo by Eli Sagor via Flickr

The impact of deer browsing is complicated. Deer populations in the study area quadrupled after measurement began in 1980. Deer herbivory suppressed growth of all plant species when their stems were thin (3 – 10 cm DBH). However, after 1996 rapid growth of plants in openings caused by Hurricane Fran’s passage began to reverse the effects of deer browsing. Also, while deer browsing decreases regeneration, growth, and abundance of oak and hickory seedlings and saplings, it also decreases the abundance of other tree species that have – nevertheless – increased in abundance, e.g., red maple (A. rubrum) and black cherry(Prunus serotina).

Payne and Peet found that soil attributes (wetness, texture, organic matter and chemical components), as well as topographic position were minor factors in determining succession trajectories. Increased light availability due to the new or exacerbated drivers of change (thinning of understory vegetation by disease and deer herbivory and opening of the canopy by hurricanes) overcame the influence of nutrients. At most, a unique soil condition might constraining the impacts of these disturbances. Furthermore, these soil-related conditions and other environmental variables change through time — and as a result so does the vegetation. Specifically, the conditions that once supported establishment of oaks and hickories apparently differ today. Payne and Peet conclude that other drivers might be continuing to impact these species’ maturation.

A partial exception is soil nitrogen, through its influence on mycorrhizal patterns. I review mycorrhizal patterns in the discussion of individual tree species, below.

How are Individual Tree Species Responding?

Oaks and hickories are not expanding as expected – either as canopy-sized trees or as seedlings / saplings in the understory. Payne and Peet agree that century-long suppression of low-intensity ground fires is probably the most significant factor in this compositional shift. This decline has been exacerbated by selective logging and deer herbivory. Hickories have established more widely, possibly because young stems have greater shade tolerance. Only plots located on sandy and acidic soils and plots with the greatest hurricane damage have moderate recruitment of oaks and hickories. Oaks and hickories on the poor soils might be aided by the types of ectomycorrhizal fungi that survive in acidic soils with relatively low nitrogen levels. In addition, these soils’ lower water retention probably impedes competition by more mesic, faster-growing, shade-tolerant species. However, even oaks and hickories that have established as seedlings or saplings only rarely progress to canopy dominance. Payne and Peet conclude that oaks might have lost competitive advantage in many of the undisturbed stands.

More mesophytic hardwoods, especially red maple (Acer rubrum), are becoming more numerous and larger – a trend seen throughout forests of the eastern United States. Damage from Hurricane Fran apparently accelerated this trend. However, red maple growth is significantly inhibited by competition from thicket-forming shrubs, especially in bottomland plots. The invasive non-native species thorny olive or oleaster Elaeagnus pungens increased dramatically following Hurricane Fran in 1996. The situation is likely to worsen: two other invasive species, Amur honeysuckle Lonicera maackii and privet Ligustrum japonicum were first detected in the Duke Forest plots in the 2013 survey.

[In New Hampshire, Ducey et al. detected an unexpected levelling off of red maple increases and decline in sugar maple (Acer saccharum); they were unable to determine a cause.]

beech-dominated understory in northern Virginia; F.T. Campbell

Another mesophytic hardwood – American beech (Fagus grandifolia) – has become very abundant in bottomland hardwood stands, especially in small-stem size classes in the understory. Beech prefers sandy soils and its ectomycorrhizal associations are apparently more tolerant of more acidic soils.

Payne and Peet mention – briefly and vaguely – uncertainty about the future of beech. The reference cited discusses the impact of beech bark disease (BBD) in the northeast. Range maps indicate that BBD is well established in the southern Appalachians along the North Carolina/Tennessee border; it has apparently not spread as far east as the study area. There is no mention of beech leaf disease (BLD), which is the primary threat to seedlings and saplings. BLD is currently known to be in northern Virginia. It is unknown whether the disease has any climatic or other barrier that would prevent its moving farther south.

Another bottomland indicator taxon that is also increasing in abundance is ash (Fraxinus species). Along with sweetgum (Liquidambar styraciflua), tulip poplar (Liriodendron tulipifera) and black cherry Prunus serotina, ash density and basal area increased dramatically in plots heavily damaged by Hurricane Fran. Payne and Peet expect most ash trees to be killed by emerald ash borer (Agrilus planipennis) by 2022. The beetle was detected in the study area in 2015. 

ash killed by EAB on Potomac lowlands; F.T. Campbell

Flowering dogwood (Cornus florida)was one of the most abundant understory species throughout the study area until the late 1980s. The species has declined by more than 80% since then due to the non-native disease dogwood anthracnose (Discula destructiva). No other species has experienced as precipitous a decline. There is now almost no regeneration in most upland sites.

A second species almost eradicated from the study area by a non-native pathogen is American elm (Ulmus americana). Its basal area in 2013 was 5% of peak levels in the 1950s. Most of this loss occurred by the 1960s, shortly after arrived of Dutch elm disease (DED) in North Carolina. A congeneric species, slippery elm U. alata, is reported to beabundant; it is somewhat resistant to DED. There is no mention of the zig-zag sawfly (Aproceros leucopoda) which has been detected in North Carolina, a few counties away from the study area. The foliage-feeding insect’s long-term impact on elm species is not yet understood.

Payne and Peet note that the study area has twice experienced loss of important components due to specialist non-native pathogens: elms and dogwoods. A third similar event looms: ash [The article does not discuss prospects for biological control.] A fourth is less certain: beech. [This numbering assumes that American chestnut and eastern hemlock were not significant components of forests in the study area.] In their view, these events demonstrate the drastic impacts such non-native organisms can have, especially when the host species is highly abundant or otherwise dominant in a specific community. The resulting shifts in community dynamics and modifications to light and water availability due to such losses, can be dramatic and long-lasting, even resulting in novel successional trajectories.

Members of the 23rd Civil Engineer Squadron/23rd Wing chainsaw a tree lying across a street in the NCO housing area- damage to piedmont North Carolina by Hurricane Fran. Photo courtesy of U.S. National Archives.

Payne and Peet also emphasize the impact of large, episodic disturbances (in their case, hurricanes). These can have widespread and long-lasting impacts on plant community dynamics. Hurricanes’ frequency, intensity, and timing relative to successional stage are key in determining their impacts on successional trajectories. E.g., strong storms that felled the even-aged pine canopy accelerated succession toward more mixed hardwoods. These changes affect biomass, diversity, competitive dynamics, and invasion by invasive plant species, especially in sites with advantageous soil conditions.

Scientists must also evaluate interactions (both reinforcing and antagonistic) between these drivers. For example, in this study deer herbivory and damage from episodic storms had opposite effects on the density of stems in the understory and therefore the future dynamics of forested stands. Hurricane aftereffects frequently accelerated existing or developing trends resulting from various other drivers (e.g., loss of dogwood to anthracnose disease). [While Ducey et al. also detected lasting impacts from hurricane damage in New Hampshire, these effects did not include changes in tree species composition.] Broader regional and global drivers of change, especially those associated with climate change and nitrogen deposition, interact with these many indicators in novel ways based on their own local loadings.

The Nature Conservancy focuses on fire

The Nature Conservancy magazine for Winter 2023 carries an article describing the organization’s experimental efforts to promote oak succession in the Piedmont forests of North Carolina. Greg Cooper, TNC’s forest ecologist in North Carolina, describes retaining dominance by oaks and hickories – rather than maples and poplars – as vital to protecting the region’s faunal diversity and minimizing impacts from climate change. He says this is because oaks use a quarter of the water of maples and poplars.

Cooper links oaks’ failure to reproduce on fire suppression. TNC kills midstory maples and poplars through hack and squirt methods. This allows more light to penetrate the forest and foster oak seedling recruitment. Then they apply controlled fire. “We currently have 700 acres of [controlled-] burn plots, some of which have been burned twice, some of which have been burned once, [and already] we’re getting more light and an immediate flush of herbaceous diversity. We’re getting a lot more berry species, more wildflowers.” TNC is monitoring plots that have been burned, with and without the pre-burn herbicide treatments, and those that have not been burned. They hope to have results in five to ten years that will indicate whether they are achieving the desired improvement in oak regeneration.  If so, they also hope is that in future prescribed burns will be sufficient.

Cooper adds that through the Fire Learning Network and a 23-person fire crew they carry out similar work not just on TNC properties, but also federal and state properties.

SOURCES

Ducey, M.J, O.L., Yamasaki, M. Belair, E.P., Leak, W.B. 2023.  Eight decades of compositional change in a managed northern hardwood landscape. Forest Ecosystems 10 (2023) 100121

Payne, C.J. and R.K. Peet. 2023. Revisiting the model system for forest succession: Eighty years of resampling Piedmont forests reveals need for an improved suite of indicators of successional change. Ecological Indicators 154 (2023) 110679

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

or

www.fadingforests.org

Imports from Asia rise; perhaps 2,000 or more containers carrying insect pests enter U.S. in one month

Wood packaging – crates, pallets, spools for wire, etc. — has been recognized as a major pathway for introduction of tree-killing pests since the Asian longhorned beetle was detected in New York and Chicago in the late 1990s. As of 2021, 65 new species of non-native wood- or bark-boring Scolytinae had been detected in the United States (Rabaglia; full citation at end of the blog).

As I have often reported [To see my 40+ earlier blogs about wood packaging material, scroll down below archives to “Categories,” click on “wood packaging”.], the international phytosanitary community adopted the International Standard for Phytosanitary Measures (ISPM) #15. The goal of ISPM#15 is to “significantly reduce” [not eliminate] the risk of pests associated with solid wood used for constructing packaging (e.g., crates, pallets), from being introduced to other countries through international trade.

I recently reviewed the first 20+ years of implementation of ISPM#15 including two analyses by Robert Haack and colleagues in a blog in December 2022. I have also provided the broader context of the World Trade Organization (WTO) in my Fading Forests II report.  

I last blogged about U.S. import volumes in June. My silence since reflected the significant decline in U.S. imports from Asia. This reduction had reduced the likelihood that a new tree-killing pest would be introduced from that region – or that an already-established pest would be introduced to a U.S. region that had escaped it so far.

However, U.S. imports from Asia have suddenly grown! In October 2023, containerized imports from Asia were 12.4% higher than a year ago – and 6% higher than in September. According to the Journal of Commerce (full citation at end of blog), U.S. retailers anticipate consumers will purchase lots of gifts for the upcoming Christmas season.

The U.S. imported 1.57 million TEU from Asia in October. This volume exceeded even the pre-COVID levels. How great is the associated risk of a pest introduction? To calculate that, I apply the following:

  • most U.S. imports arrive in 40-foot-long containers, so divide TEU by 2 = 785,000
  • a decade-old estimate that 75% of containers in maritime shipments contain wood packaging (Meissner et al.) = 588,750 containers with wood packaging (I suspect it is more).
  • the estimate by Haack et al. 2014 that 0.1% (1/10th of 1 percent) of consignments (which usually means a single container) harbor tree-killing pests;
  • the estimate by Haack et al. 2022 that 0.22% of consignments harbor tree-killing pests.
inspecting a pallet; CBP photo

The result of these calculations is an estimate of 648 containers (using the 2009 global estimate), or 1,727 containers (using the 2022 global estimate), or 5,730 containers (using the 2010-2020 estimate for China specifically) entering the country in one month harbored tree-killing pests. Since West Coast ports received 54% of those containers, the estimated number of containers transporting pests that enter California, Washington, or Oregon ranged from 349 to 3,042. The rest are scattered among the dozens of ports on the East and Gulf coasts.

With drought limiting container ship transits through the Panama Canal (Szakonyi 2023), the threat to East and Gulf coast ports might not rise commensurately.

Because of the low levels of imports in previous months, U.S. imports from Asia remain significantly below levels in previous years: 16.6% lower for the January – September period compared to 2022.

The 2022 analysis found that the rate of wood packaging from China that is infested has remained relatively steady since 2003: 1.26% during 2003–2004, and ranged from 0.58 to 1.11% during the next three time periods analyzed. Packaging from China made up 4.6% of all shipments inspected, but 22% of the 180 consignments with infested wood packaging. Thus the proportion of Chinese consignments with infested wood is five times greater than would be expected based on their proportion of imports.  Note the great impact of this high infestation rate on the number of containers transporting tree-killing pests to the U.S. in the paragraph above: more than 8,000 containers compared to about 2,000.  

I remind you that the U.S. and Canada have required treatment of wood packaging from China since December 1998. Why are the responsible agencies in the United States not taking action to correct this problem? [which has persisted for 2 decades]

The fact is – as I have argued numerous times — a pallet or crate bearing the ISPM#15 mark has not proved to be a reliable indicator as to whether the wood is pest-free. (This might be because the wood had not been treated, or if it was, the treatment failed). All the pests detected in the Haack et al. studies (after 2006) were in wood packaging bearing the ISPM#15 mark. As noted in my past blogs [click on the “wood packaging” category to bring up blogs about wood packaging and enforcement], Customs and Border Protection also report that nearly all the wood packaging in which that they detected insect pests bore the ISPM#15 mark.

According to Angell in November (full citation at end of blog), U.S. imports from India to the east coast fell by 15% in the first 10 months of 2023 compared to last year – to a total of 623,356 TEUs. This might change in the future: a shipper has promised to start weekly arrivals from India beginning in May 2024. the company plans calls at New York-New Jersey, Savannah, Jacksonville, Charleston, and Norfolk. The ships will call, en route, at ports in Saudi Arabia, Egypt, and Spain. What pests might be hitching a ride on these shipments?

SOURCES

Haack RA, Britton KO, Brockerhoff EG, Cavey JF, Garrett LJ, et al. 2014. Effectiveness of the International Phytosanitary Standard ISPM No. 15 on reducing wood borer infestation rates in wood packaging material entering the United States. PLoS ONE 9(5): e96611. doi:10.1371/journal.pone.0096611

Haack RA, Hardin JA, Caton BP and Petrice TR. 2022. Wood borer detection rates on wood packaging materials entering the United States during different phases of ISPM#15 implementation and regulatory changes. Frontiers in Forests and Global Change 5:1069117. doi: 10.3389/ffgc.2022.1069117

Meissner, H., A. Lemay, C. Bertone, K. Schwartzburg, L. Ferguson, L. Newton. 2009. Evaluation of pathways for exotic plant pest movement into and within the greater Caribbean Region.  

Mongelluzzo, B. 2023. U.S. imports from Asia hit 2023 high in October despite muted peak season. Journal of Commerce https://www.joc.com/article/us-imports-asia-hit-2023-high-october-despite-muted-peak-season_20231116.html (access limited to subscribers, unfortunately)

Angell, M. 2023. ONE readies India-US East Coast service as part of 2024 network rollout. Journal of Commerce. November 27, 2023

Rabaglia, R. 2021. The increasing number of non-native bark and ambrosia beetles in North America. International Union of Forest Research Organizations. Prague, Czech Republic. September 2021

Szakonyi, M. 2023. Carriers Weigh Options as Panama Canal restrictions become fact of life. Journal of Commerce. November 21, 2023. (Access limited to subscribers, unfortunately)

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

or

www.fadingforests.org

USFS Forest Health Protection program: what it funds

affects of mountan pine beetle on lodgepole pine in Rocky Mountain National Park, Colorado photo from Wikimedia; one of pests addressed by USFS FHP

Several USFS scientists have published an assessment of the agency’s program to enhance forest health across the country: the Forest Health (FHP) program. [see Coleman et al., full citation at end of this blog.] The program assists cooperators (including other federal agencies) to prevent, suppress, and eradicate insect and pathogen outbreaks affecting trees, regardless of land ownership.

Each year, I advocate for adequate funding for the FHP program — which comes from annual Congressional appropriations. Funding has remained static at about $100 million per year. I interpret the article as providing support for my call for increased appropriations. First, it reports that the number of projects and extent of area treated have declined from 2011 to 2020. This is because static funding levels are stretched increasingly thin as costs to implement the same activities rise. Second, the program does not address many damaging forest pests already in the country. The result is growth of established threats to forest health. Finally, new insects and pathogens continue to be introduced. Protecting forest health necessitates tackling these new pests – and that requires money and staff.   

Coleman et al. analyzed data from the decade 2011- 2020 to determine the most frequently used project types, integrated pest management (IPM) strategies and tactics, dominant forest pests and associated hosts managed, and most comprehensive forest IPM programs in practice. While there is a wide range of possible projects, most of those funded consist of some form of treatment (more below). The databases relied on do not include funding through the National Forest System aimed at improving forest health through such  management activities as stand thinning treatments and prescribed fire. Nor are all pest management activities recorded in the centralized databases. I regret especially the fact that “genetic control” (= resistance breeding) are left out.

Port-Orford cedar seedlings in trial for resistance to Phytophthora lateralis at Dorena center; photo courtesy of Richard Sniezko, USFS

Summary of Findings

The data are sorted in various categories, depending on whether one wishes to focus on the type of organism being managed or the management approach. All presentations make evident a dramatic imbalance in the projects funded. Again and again, spongy moth (Lymantria dispar dispar), southern pine beetle (SPB, Dendroctonus frontalis), and several bark beetles attacking conifers in the West (in particular mountain pine beetle, [MPB] Dendroctonus ponderosae) dominate, as measured by both funding and area treated.

oak trees in Shenandoah National Park killed by spongy moth; photo by F.T. Campbell
  • The bulk of the funding went to the above species, plus hemlock woolly adelgid (HWA; Adelges tsugae); emerald ash borer (EAB, Agrilus planipennis), oak wilt (caused by Bretziella fagacearum), and white pine blister rust (WPBR, Cronartium ribicola).
  • 95% of the projects focused on only four taxa: oaks, Quercus spp. [spongy moth suppression and eradication]; loblolly and ponderosa pines [bark beetle prevention and suppression]; and eastern hemlock [HWA suppression].
  • Projects seeking to suppress an existing pest outbreak covered 87% of the total treatment area. However, 98% of the treated area was linked to only 20 taxa; again, spongy moth dominated.
  • Projects seeking to prevent introduction or spread of a pest constituted only 30% of all projects and covered only 11% of the total treatment area.
  • Eradication and restoration projects each equaled less than 5% of total projects and treatment areas.
  • Native forest pests were targetted by 79% of projects; non-native pests by 21%. However, non-native pests accounted for 84% of the total treatment area (again, the spongy moth).
  • While 67% of projects took place on USFS lands (focused on MPB and SPB), 89% of the total treatment area was on lands managed by others (state or other federal agencies, or private landowners). Again, the size of the non-USFS  area treated was driven primarily by the spongy moth Slow the Spread program.
  • Insect pests received nearly all of the funding: 70% of funding targetted phloem-feeding insects, especially SPB and MPB; 10% targetted foliage feeders, especially spongy moth; 6% targetted sap feeders. 4% tackled rusts (e.g., WPBR); just 2% addressed wood borers (e.g., Asian longhorned beetle, emerald ash borer).
  • The ranking by size of area treated differs. In this case, 82% of areas treated face damage by foliage feeders (e.g., spongy moth); 15% of the treated areas are threatened by phloem feeders (e.g., MPB); only 1.4% of the area is damaged by sap feeders (e.g., HWA); 0.6% is threatened by rust; and 0.2% by wood borers.
  • Re: control strategies, 32% of projects relied on silvicultural strategies; 22% used semiochemical strategies; 21% exploited other chemical controls; and 18% used physical/mechanical control methods.

Coleman et al. regretted that few programs incorporated microbial/biopesticide control strategies; these were applied on only 10% of total treated area. Again, the vast majority of such projects were aerial applications of spongy moth controls, Bacillus thuringiensis var. kurstaki (Btk) and nucleopolyhedrosis viruses (NPV) (Gypchek). Coleman et al. called for more research to support this approach efforts to overcome other obstacles (see below).

Coleman et al. also called for better record-keeping to enable analysis of genetic control/ resistance breeding projects, treatment efficacy, and survey and technical assistance activities.

History

The article provides a brief summary of the history of the Forest Service’ pest management efforts. Before the 1960s, the USFS relied on labor-intensive physical control tactics, classical biocontrol, and widespread chemical applications. Examples include application of pesticides to suppress or eradicate spongy moth; decades of Ribes removal to curtail spread of white pine blister rust; salvage logging and chemical controls to counter phloem feeders / bark beetles in the South and West. These strategies were increasingly replaced by pest-specific management tactics during the 1970s.

Over the decade studied (2011-2020), tree defoliation attributed to various pests (including pathogens) affected an estimated 0.7% of the 333 million ha of U.S. forest land annually. Mortality attributed to pests impacted an estimated 0.8% of that forest annually. See Table 1. Two-thirds of the area affected by tree mortality is attributed to phloem feeders; a distant second agent is wood borers. These data are incomplete because many insects, diseases, and parasitic higher plants are not tracked by aerial surveys.

As I noted above, these data do not include projects that screen tree species to identify and evaluate genetic resistance to a pest; or efforts to collect cones, seed, and scion. I consider these gene conservation and resistance programs to be some of the most important pest-response efforts. I have blogged about the USFS’ Dorena Genetic Resource Center’ efforts to breed five-needle pines, Port-Orford cedar, and ash. link

41% of silvicultural control treatments targetted phloem feeders; 48% addressed cankers and rusts together. Restoration planting was done in response to invasions by ALB, EAB, and WPBR, as well as native bark beetles and mistletoes.

effort to eradicate SOD in southern Oregon; partially funded by USFS FHP. Photo courtesy of Oregon Department of Forestry

Physical/mechanical control projects were most widely applied in the Rocky Mountains in response particularly to diseases: vascular wilts, rusts, and cankers, including WPBR. This type of project was also used to deal with non-native diseases in other parts of the country, e.g., oak wilt, sudden oak death (SOD), Port-Orford cedar root rot, and rapid ʻōhiʻa death. Sanitation treatments (i.e., removal of infected/infested trees) was used for native mistletoes and root rots, and some non-native insects, e.g., EAB and coconut rhinoceros beetle (Oryctes rhinoceros). Pruning is a control strategy for WPBR. Trenching is applied solely to suppress oak wilt.

Chemical controls were limited to small areas. These projects targetted seed/cone/flower fruit feeders, foliage and shoot diseases, sap feeders [e.g., balsam woolly adelgid (BWA), HWA], wood borers (e.g., EAB) and phloem feeders (e.g., Dutch elm disease; DMF oak wilt vectors). Cover sprays have been used against goldspotted oak borer (GSOB); and many native insects. Fungicides are rarely used; some is applied against the oak wilt pathogen in areas inaccessible by heavy equipment.

treating hemlock trees in Conestee Falls, NC; photo courtesy of North Carolina Hemlock Restoration Initiative

Classical biocontrol projects funded by the program targetted almost exclusively HWA. Some 4.3 million predators have been released since the early 1990s; 820,057 in just the past 10 years.

Gene conservation and breeding projects were directed primary at commercially important hosts, e.g., loblolly Pinus taeda and slash pine P. elliottii; and several non-native pests, including chestnut blight, EAB, HWA, and WPBR.

Survey and technical assistance (i.e., indirectly funded activities) conducted by federal, state, and tribal personnel contributed to education/outreach, evaluating effectiveness, identification, monitoring, and record keeping strategies.

As should be evident from the data presented here, suppression treatments dominated by number of projects and treatment area. The poster child project is the national spongy moth Slow the Spread program. The authors say this program is the most advanced forest IPM program in the world. It has successfully slowed spongy moth’s rate of spread by more than 80% for more than 20 years.

A second widely-used subset of suppression programs consists of physical / mechanical control. This is often the principal suppression strategy in high-visitation sites (e.g., administration sites, campgrounds, picnic areas, and recreation areas). Sanitation harvests are one of the few viable management techniques for suppressing or slowing the spread of recently introduced non-native pests. Nevertheless, the largest number of suppression projects and use of sanitation treatments focused on a native pest, mountain pine beetle, at the height of its outbreak in early 2010s.

Silvicultural control, specifically tree thinning, represents the predominant forest pest prevention tactic, especially on lands managed by the USFS. Two programs dominate: the Southern Pine Beetle Prevention Program and the Western Bark Beetle Initiative. Again, Coleman et al. assess these treatments as very successful. Forest thinning treatments also address other management concerns, i.e., reduce threat of catastrophic wildfires and reduce adverse effects of climate change.

Chemical control tactics are applied to suppress most forest insect feeding guilds in high-value sites and seed orchards. Soil or tree injections of systemic pesticides are used to protect ash and hemlock trees. Topical sprays have been applied to protect whitebark pine (Pinus albicaulis) from mountain pine beetle. Whitebark pine was listed as threatened under the Endangered Species Act in December 2022.

dead whitebark pine at Crater Lake NP; photo by F.T. Campbell

Soil or tree injections target two non-native insects, EAB and HWA.

Genetic control via resistance breeding represents the primary strategy to combat several non-native diseases. (More options are typically available for insects than diseases.) Coleman et al. focus on the extensive effort to protect many of the five-needle pines from WPBR. As I have described in earlier blogs, the Dorena Genetic Resource Center in Oregon has engaged on numerous other species, too.

Coleman et al. describe pest-management associated monitoring efforts as consisting largely of coordinated annual aerial detection surveys, detection trapping, stream-baiting of Phytophthora ramorum, and ground surveys to address site-specific issues.

Coleman et al. call for improvement of record-keeping / databases to encompass all pests, management actions, and ownerships. They also advocate for additional decision-making tools, development of microbial/biopesticides, genetic research and breeding, and biocontrol strategies for several pest groups.

They consider the southern pine beetle and spongy moth programs to be models of comprehensive IPM programs that could be adapted to additional forest health threats. They note, however, that development and implementation of these programs require significant time, financial commitments, and collaborations from various supporting agencies. Not all programs enjoy such resources.

SOURCE

Coleman, T.W, A.D. Graves, B.W. Oblinger, R.W. Flowers, J.J. Jacobs, B.D. Moltzan, S.S. Stephens, R.J. Rabaglia. 2023. Evaluating a decade (2011–2020) of integrated forest pest management in the United States

Journal of Integrated Pest Management, (2023) 14(1): 23; 1–17

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

or

www.fadingforests.org

New APHIS risk assessment concludes eastern forests safe from SOD. I note too many uncertainties

northern red oak – host of P. ramorum in eastern U.S. forests Photo by F.T. Campbell

The 2023 USDA APHIS risk assessment (PRA) seeks to analyze the possibility that the pathogen will invade and damage forests outside of California and Oregon – especially the deciduous forests of the East.

Is this study preparation for ending federal regulation of this pathogen through the nursery trade? If so, I think this study warrants particularly careful scrutiny. I raise questions about the study’s assumptions and the admitted uncertainties affecting several critical issues.

Managing Phytophthora ramorum – the causal agent of sudden oak death and ramorum blight – has proved to be difficult. It has demanded considerable resources over more than 20 years. I certainly agree that APHIS should focus on real risks. However, I am not satisfied that this risk assessment sufficiently evaluates the risk posed by P. ramorum.

APHIS considers the risk assessment to be final now, after receiving comments from state phytosanitary agencies – via the National Plant Board – and the U.S. Forest Service. APHIS is not seeking additional input. This is unfortunate given the unanswered questions and notable gaps in the study. (Details below.)  Also, the agency has drafted an implementation plan, which staff hope to issue in the coming months. (Congress’ failure to complete Fiscal Year 2024 appropriations, and continuing disagreements about how to proceed, will probably delay this.)

The Risk Assessment’s Major Conclusions

The APHIS assessment concludes that Phytophthora ramorum probably will not cause significant disease in forests outside of California and Oregon. This is because the infective stage of P. ramorum and the susceptible stage of the host complex do not occur at the same time in eastern forests (as they do in the currently infected range). That is, the infectious agent is not present during the period when environmental conditions are favorable for infection, disease development, and spread.

1) Environmental stress, such as heat, decreases survival of P. ramorum inoculum.

2) As a result, inoculum does not build up sufficiently to produce significant disease.

3) The infectious stage of the P. ramorum (zoospore production) does not overlap long enough with the susceptible stage of the host or host complex for disease to develop.

The PRA concludes that while P. ramorum might survive in these environments, disease will not develop. Hosts will not become symptomatic “at a noticeable scale.” (page i of the PRA)

oak forest in West Virginia; photo by ForestWander via Flickr

The authors also conclude that it is unlikely that repeated incursions of the pathogen or changes in climate conditions could increase inoculum pressure sufficiently to cause infection plus disease. The former is especially relevant because infected nursery plants have been shipped to eastern states repeatedly. The assessment lists at least 20 episodes since 2004 (Table 1). In total, P. ramorum has probably been moved more than a thousand times on nursery stock from California, Oregon, or possibly other states (or British Columbia).

The Risk Assessment notes three important sources of uncertainty that affect one or more of its major conclusions:

1) Little is known about the susceptibility and competency of host plant species in the eastern U.S. “Host competency” is the ability of a host species to transmit the infection to another susceptible host or to a vector. This is assessed by measuring pathogen sporulation, production of sporangia or zoospores on the various host species. (Discussed in greater detail below.)

2) Some climatic factors important for disease development cannot be reliably modeled for forest conditions. Until this changes, it seems to me that findings regarding infections and climate change are questionable.

3) A host range expansion due to the introduction or evolution of new clonal lineages might increase the adaptability of P. ramorum in the U.S. and potentially alter the consequences of introduction. Such shifts are definitely possible. In Europe, the EU1 clonal lineage has infected Japanese larch (Larix kaempferi). Both the EU1 and an additional strain of P. ramorum (NA2) have been established in the forests of Oregon for seven years, in California for a few years less. Establishment of the EU1 lineage also increases the chances for sexual reproduction, genetic recombination, and altered biology and epidemiology of this pathogen.

[Some scientists reached the opposite conclusion, although they did not delve as deeply into the climatic factors; instead they focused on host presence in wide climate ranges. See Haller and Wimberley 2020; full citation at end of this blog.]

Description of SOD’s Impact in the West

The PRA provides a decent summary of the history of Phytophthora ramorum in the U.S. It includes dates of detections; how the link was made between the disease and the newly discovered pathogen; its disease cycle; and the principal hosts in California and Oregon. It also documents the tens of millions of trees killed in California and Oregon, along with the resulting changes in forest composition and structure, threats to dependent wildlife species, and increasing fire risks. The study notes the threat to manzanita (Archtostaphylos). California is the center of diversity for the genus, and 59 out of the 105 species that inhabit the state are rare or endangered species.

APHIS: No disease in Eastern Forests Despite Frequent Exposure

As noted above, over the 25+ years since P. ramorum was first discovered in California (and later Oregon) forests and nurseries, infected plants have been shipped to nurseries throughout the country perhaps 1,000 times. Every one of these shipments was in violation of federal regulations conceived, adopted, and implemented by APHIS.

Despite the frequent arrival of P. ramorum-infected plants in nurseries, the less frequent planting of these plants in private and public gardens (many infected plants are destroyed once the infection is detected), and the persistence of detectable P. ramorum spores in streams in the southeastern states, the pathogen has not been detected causing disease in the environments of states other than California and Oregon. The PRA says these 20 years of experience indicate that development of significant disease is unlikely even if propagule pressure increases.

What Worries Me

The PRA states that APHIS scientists’ concept and evaluation of risk have “evolved.” The PRA does not explain this change explicitly; I would have appreciated a discussion of this change.

There is no evidence at present of sustained infectious outbreaks in the forests of the eastern states or Washington State.  However …

  1. The PRA does not even summarize the dozens of known hosts native to eastern deciduous forests. Nor does it report findings of previous laboratory studies regarding the hosts’ capacity to sustain a disease epidemic. Instead, the PRA dismisses these studies with the statement “except for some eastern forest understory species [citing various studies by Paul Tooley and others], we do not have a good understanding of host transmission and susceptibility for tree and shrub species outside of California and Oregon. For this reason we do not include hosts in these maps.”

Since the purpose of the study is to evaluate the risk to those eastern forest species, shouldn’t the authors describe what is already known? Also, the risk assessment lacks an analysis of  the gaps in our knowledge (beyond saying  that research studies cannot be compared due to different methods).

  • Furthermore, the risk assessment is never clear about which species in eastern forests the authors consider important. Are they concerned about the possible mortality only of canopy-sized oaks (Quercus spp.)? Do they consider the threat to shrubs and sub-canopy trees such as dogwood (Cornus spp.), sassafras (Sassafras albidum), andmountain laurel (Kalmia latifolia)? Or do they rate these species important only as possible drivers, not victims, of disease? (See below.)
Kalmia latifolia (cultivar); photo by F.T. Campbell
  • The PRA stresses the importance of climate in disease transmission – specifically relative humidity and the timing of rain events. In the Mediterranean climate of coastal California, this means ample rainfall and mild temperatures in late spring. The PRA provides no data documenting that timing is as critical in the cool and humid climate of Oregon and far northern coastal California, or the United Kingdom. These are the areas where P. ramorum has caused the most serious disease. Portions of the eastern deciduous forest – e.g., mid-elevations of the southern Appalachians – might more closely resemble humidity and temperature conditions of Oregon and Britain than central California. If so, timing might be less decisive a factor there, too.

Temperature is also important: P. ramorum thrives in regions with a 20°C difference between the minimum and maximum daily temperatures. The infectious stages are produced in a relatively narrow temperature range: sporangia between 16 and 22°C, zoospores between 20 and 30°C. The PRA says that after mild temperatures, the second most predictive factor varies by the genetic strain of the pathogen: for the NA1 and NA2 strains, it is minimum temperature of the driest month; for the EU1 strain, it is precipitation during the driest month.

The PRA concludes that areas in the East where temperatures are suitable for infection are rare – see Figure 5B in the document. However …

P. ramorum usually reproduces asexually. When temperature and humidity conditions are conducive, the P. ramorum mycelium produces zoospores which then spread the infection by swimming to new infection sites. Persistence of the crucial film of water on leaves depends on the microclimate of specific sites. Sites that are cooler and damper are present in many parts of the eastern forest, e.g., stream canyons, upper elevations, north-facing slopes. Several of the known or suspected hosts in the east, e.g., Kalmia and Rhododendron species and hemlocks, grow in such sites. Those who purchase plants of these genera often maintain outdoor plantings in or near such sites. Therefore, I continue to worry that conducive conditions might be present in nurseries and gardens of private homes in mountainous areas.

  1. The PRA concedes assessors had insufficient data at scales useful to model the risk associated with these microclimates, or to separate individual effects of temperature, rain, and leaf wetness on P. ramorum infection. I object to glossing over this ignorance. Absence of evidence is not equal to evidence of absence – especially when such iconic and valuable plants are involved.

The Risk Assessment concludes that eastern forests will frequently have inhospitable conditions. This will prevent development of enough spores to initiate widespread disease. However, P. ramorum survives as abundant chlamydospores during conditions such as hot summers, cold winters, and drought. These cells germinate when conditions become suitable. As I noted just above, important portions of eastern deciduous forests provide such conducive conditions – and APHIS says it is unable to model these microclimates.

P. ramorum can also survive in decomposing leaf litter in water. The pathogen has regularly been detected in nearly a dozen streams in southeastern states. In APHIS’ assessors’ opinion, the oomycete cells in these conditions are unlikely to produce sporangia and zoospores that can infect living host tissue. P. ramorum can’t complete its lifecycle without infecting a living plant and the chain of infection will break.

2. The PRA concedes considerable uncertainty associated with the possibility of sexual reproduction. While sexual reproduction has rarely occurred to date, this is probably because the two mating types have been located on two continents: isolates of the A1 mating type have been in only Europe until recently. The A2 isolates are in North America.

This barrier has now been breached. Since 2016 or earlier, a European strain of the A1 mating type (the EU1 lineage) has been established in Oregon and – more recently – in California, near populations of the North American A2 mating type (NA1). The NA2 lineage is also established in Oregon’s forest. The EU1 genotype produces prolific spores in Europe and has the potential to be more aggressive than either North American strain (NA1 and NA2). The presence of EU1 introduces the possibility for sexual reproduction through crossing with the NA1 or NA2 lineages. Indeed, an EU1 – NA2 hybrid has been discovered in a Canadian nursery (that outbreak has reportedly been eradicated). The EU1 strain is currently restricted to western forests. However, it might spread to nurseries, which seem likely to ship EU1-infected plants to eastern states.

Additional genetic variation is probable through introduction of yet more strains, given the failure of current phytosanitary measure to prevent continuing introductions of the pathogen. Scientists recently discovered eight additional strains of P. ramorum in Southeast Asia [the PRA cites Jung et al., 2021]. Each strain represents different adaptations and presents the opportunity for sexual reproduction that might facilitate adaptation to new conditions. The Vietnamese P. ramorum strains are found at high elevations. In the central highlands, at least, rainfall occurs primarily in the summer (the opposite pattern from California). The average annual temperature is 21 to 23°C (70 to 73°F). Winter mean temperatures can fall below 20°C (68oF). I believe the PRA should have discussed how these climatic attributes compare to parts of the eastern U.S. that could be at risk.

Finally, the PRA notes that asexual populations of P. ramorum can “jump” hosts, e.g., EU1 and EU2 in Europe now attack Japanese larch. The EU1 strain in Oregon is infecting grand fir (Abies grandis) and Douglas-fir (Pseudotsuga menziesii) and can occur in the right environmental conditions. The assessment does not discuss the level of disease on these conifers. Perhaps it is too early in the disease progression to know, but the prospects are worrying.

3. As this Risk Assessment notes, in California and Oregon only some hosts contribute to disease spread by P. ramorum. In those states, transmissive hosts are California bay laurel (Umbellularia californica) and tanoak (Notholitocarpus densiflorus). These sporulating hosts drive the epidemic. The true oaks (Quercus spp.) are dead-end hosts – they develop lethal infections but do not contribute to disease spread. Tanoak is unique in being both a sporulating and a mortally vulnerable one. The study seems to assume that the disease will behave similarly in the East. That is, canopy-forming oaks and other trees will be killed but will not transmit infection. The assessment provides no basis for this assumption. I agree that transmissive hosts must be sufficiently tall to allow spores to spread to other hosts through downward rainfall or wind. Do we know that larger eastern oaks, are “dead-end” hosts?What happens if eastern conifers become infested? For example, eastern hemlocks? What about hosts that climb into the canopy, e.g., Japanese honeysuckle (Lonicera japonica)? The risk assessment does say that Rhododendron and Kalmia can reach heights comparable to California bay laurel and that in the United Kingdom disease is sustained by infected Rhododendron. I note that Rhododendron and Kalmia grow in high-elevation “balds” in the Appalachians – where spores could be picked up by wind-driven rain.

Several of the eastern hosts are already – or soon will be – threatened by other non-native pests. A little to the east of the Great Smokey Mountains dogwood’s density is now a fifth of the peak registered 30 years earlier. There is now almost no regeneration in most upland sites. Dogwood anthracnose is more virulent in cool, damp environments – microclimates most suitable for  SOD. Hemlocks have been decimated by hemlock woolly adelgid, opening formerly cool, dark environments to more sunlight. Sassafras faces great losses to laurel wilt disease. These other non-native pests and pathogens might reduce the likelihood of P. ramorum encountering sporulating hosts. On the other hand, these existing threats raise the importance of protecting these hosts from impacts from yet another invasive species.

eastern (flowering) dogwood; photo by F.T. Campbell

Most alarming is the fact that the PRA assessors admit they don’t understand how climatic conditions affect host susceptibility or which eastern forest species might be transmissive hosts. The absence of such call their assumptions and findings into question. An APHIS staffer reports that scientists are now working on these issues. I believe APHIS should have begun studying these issues years ago, given the frequency of the spread many pests via the nursery trade. At a minimum, they should not have published a risk assessment until these questions have been resolved.

Included among the topics the PRA says need additional research are issues underlying vulnerability of eastern forest species and ecosystems:

• Susceptibility and competency of the various host combinations in both eastern and western forests.

• Timing of inoculum production of the various host combinations in the potential host complexes, and whether timing of inoculum production overlaps with when hosts would be susceptible.

• Timing and duration of various climatic factors favorable for infection.

The document does not indicate whether APHIS has funded these research studies. We have only the brief personal statement by the APHIS program leader, above.

The PRA complains about the lack of quantifiable data on the consequences of ramorum blight in nurseries. After noting unmitigated presence of P. ramorum will damage leaves and cause shoot dieback and damping off (death) of young plants, the assessment concludes that scientists and managers understand nursery practices that are effective in mitigating P. ramorum and ramorum blight. However, the assessors are uncertain re: the extent to which nurseries already routinely apply these practices. I add: or will continue to do so if regulatory requirements are ended.

The authors say estimating environmental consequences in other parts of the country is difficult because susceptibility of eastern species remains unclear. This is especially the case with the possibility of P. ramorum infecting conifers and the increased likelihood of sexual reproduction – at least in the West. The study then concludes that unless conditions change, P. ramorum does not pose a high risk to the forests of the rest of the US. Given all these unknown, I consider this conclusion to be unwarranted.

Might the States Substitute for APHIS?

If APHIS ends its regulatory program for P. ramorum, states will be free to adopt their own regulations (see Porter and Robertson, 2011; and Fading Forests III, link at end of this blog). Under the best circumstances, states will coordinate such a response – as they did earlier for thousand cankers disease of walnut. However, the absence of a federal regulation will still raise the (already too high!) likelihood that infected plants will be shipped to eastern states.

SOURCES

Haller, D.J. and M.C. Wimberly. 2020. Estimating the Potential for Forest Degradation in the Eastern United States Woodlands from an Introduction of Sudden Oak Death. Forests 2020, 11, 1334; doi:10.3390/f11121334 www.mdpi.com/journal/forests

Porter, R.D. and N.C. Robertson. 2011. Tracking Implementation of the Special Need Request Process Under the Plant Protection Act. Environmental Law Reporter. 41.

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

or

www.fadingforests.org