On the first day of winter, Daniel Volk, Forest Health Project Coordinator for Cleveland (OH) Metroparks reported that a coordinated survey has confirmed the presence of beech leaf disease link to DMF in four new states — Massachusetts, New Jersey, Rhode Island, and West Virginia. In all, the disease is now known to be established in 71 counties in the US and Canada. Funding was provided by the USFS Forest Health Management “emerging pest” program.
2021 survey efforts will focus on high risk counties adjacent to affected counties.
Cleveland Metroparks has several resources available on its website and will continue to post updated information there as it becomes available.
I posted a blog urging recipients to participate in these searches last June link I hope you will do so again in 2021.
range of American beech
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
In August I blogged that import volumes had crashed … US imports from Asia declined each month from January through June (Mongelluzzo Dec 14; full citations at end of blog). However, the economic rebound over the summer brought a surge in imports that continues. Given our concern about introductions of tree-killing pests, it is not good news that imports from Asia are driving the growth in imports.
US imports from Asia during the first 11 months of 2020 were 2% higher than the figure from the first 11 months of 2019. In November alone, the U.S imported 1.6 million TEUs [twenty-foot equivalent units; a standard measurement that counts incoming volume as though contained in twenty-foot-long containers] worth of goods from Asia. Imports in December are projected to remain high (Mongelluzzo Dec 14; full citation at end of blog).
Imports from Asia were 1.626 million TEU in December – up 29.9% from December 2019 (although still lower than October and August). December imports were pushed by record e-commerce sales and shipments of personal protective equipment (PPE) and medical supplies. For all of 2020, imports from Asia totaled 16.6 million TEU, up 4.1% from 2019 (Mongelluzzo Jan. 19)
This surge in imports – which began in late June — is certain to continue at least for the next two months as retailers ship more merchandise before some factories in Asia close for the Lunar New Year (Mongelluzzo Jan 19).
Because of the history of tree-killing pests introduced from Asia, I have blogged most often about the situation at West Coast ports. However, in 2020 there has been a noticeable shift to East and Gulf Coast ports because of the congestion and delays at West Coast ports. Thus, in November 2020, West Coast ports handled 60.2% of imports from Asia; East Coast ports handled 33.7%; Gulf Coast ports handled 5.7%. The East Coast figure is 30% higher than over the same period in 2019. At New York-New Jersey specifically, the increase was 35.1% (Mongelluzo Dec 16). Imports to Gulf Coast ports continue to rise; Gulf Coast ports handled only 4.8% of total US imports from Asia during the first nine months in 2019 and less than 3% before the widening of the Panama Canal (in 2016) (Angell October 28). Link to blog #203 midNov (In future, goods shipped from Asia across the Arctic Ocean to the U.S. east coast could add to the pest risk confronting our already hard-hit Eastern Deciduous Forest.)
Pacific Coast Ports
According to Mongelluzzo (December 9), the Los Angeles-Long Beach port complex (LA-LB) set records for US imports from Asia in August and again in October. The port complex handled 2.5 million TEU of imports from Asia in the three-month period of August through October. Despite shippers’ concern about delays, LA-LB is expected to continue to handle the bulk of Asian goods entering the country in coming months.
The ports of Los Angeles-Long Beach handle 50% of US imports from Asia. From July 2020 through February 2021, these ports received an average of 791,838 TEU each month – a 23% increase over the 2019 average of 642,000 TEU per month (Mongeluzzo April 2021).
Ports in the Southeast
As reported by Ashe (December 10), several ports in the southeast US are seeing record import volumes caused by retailers’ restocking, e-commerce, and Christmas shopping. November import volumes hit all-time highs in Savannah and Port of Virginia, while they were up year over year in Charleston. The three port authorities say the surge is the result of demand for furniture, bedding, refrigerators, freezers, and air conditioners – reflecting Americans’ current focus on improving their homes. Imports also include artificial Christmas trees (which have been a vector of pests in the past – as has furniture).
offloading cargo at Savannah; photo by F.T. Campbell
The volume of imports into Savannah from all sources surged 34% over the November 2019 volume. Imports from Asia rose 36%. Imports of furniture rose 42% in August and September. “Hardware, home goods, machinery, and appliances from Asia were up double digits,” according to Georgia Port Authority CEO Griff Lynch. Import volumes from Asia rose 36% in Virginia and 32% in Charleston.
Vessels Carry More Containers
Another threat of increased pest introductions arises from the increasing size of container ships. Increasing proportions of vessels with the capacity to carry more than 10,000 containers are arriving. Since 2010, the proportion of such ships arriving at West Coast ports has risen from 1.1% to 75.5%. The proportion arriving at East Coast ports has grown since the opening of the widened Panama Canal in 2017. The proportion of high-capacity ships visiting East Coast ports has risen from 3% in 2017 to 15% during the first 10 months of 2020. Gulf Coast ports receive few such vessels because the serve a smaller share of the U.S. market. The largest ships serve the trade from Asia primarily (Mongelluzzo Dec. 21, 2020). Of course, arrival of ten to fifteen thousand containers at once surely strains Custom’s inspection staff.
container ship in Savannah; Photo by F.T. Campbell
Imports from Geographic Regions Other Than Asia
Imports (from all sources) through New York and New Jersey ports were 22% percent higher in October 2020 than in October 2019 (Angell November 10). As noted above, most of the higher volume of imports originated in Asia.
According to Journal of Commerce staff (November 30), containerized imports from the Caribbean and Central America grew a negligible 0.1% over the same period last year. Principal ports for this trade are in Florida and along the Gulf Coast, but include Wilmington, DE, and Philadelphia.
According to JOC staff (November 2), containerized cargo import volumes from all regions flowing through the busiest US Gulf Coast ports declined 2.3% in the first seven months of 2020 compared to the same period in 2019.
Non-containerized cargoes — i.e., dry bulk, liquid bulk, roll-on/roll-off (ro-ro), and oversized/heavy-lift freight — are not included in these data. Dry bulk cargo through Houston has been reported to suffer problems in infested dunnage (wood used to brace non-containerized cargo, such as steel beams). Link to blog 173 February 2020
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
dwarfed Ulmus davidiana; photo by Krzsztof Zianek; Wikipedia Commons
USDA APHIS is seeking public input on a risk assessment that is intended to evaluate the risk of allowing importation of dwarf elm trees (bunjae) from South Korea. Importation of these trees is currently prohibited under APHIS’ authority to require a risk assessment before importation under the NAPPRA program. Upon receiving the Korean request, APHIS must decide whether to maintain the prohibition, or alter it. The risk assessment can be obtained here. Comments are due January 11, 2021.
I urge those with expert knowledge about phytophagous insects, nematodes, and fungal and other pathogens to prepare your own comments to APHIS.
[A year ago, Korea sought permission to export dwarfed maple trees to the U.S. CISP commented on APHIS’ risk assessment at that time; see my blog here. I believe APHIS has not yet decided whether to allow such imports. Many of the same issues apply here.]
After reviewing the risk assessment, I conclude that there are too many high-risk pests to support removing the taxon from the current restrictions. The history of introductions on dwarfed trees in the past supports this conclusion. The most conspicuous is the citrus longhorned beetle (Anoplophora chinensis) – the reason for the original NAPPRA listing – but there have been others, too.
The risk assessment has some strengths. I applaud the assessors for noting in each pest review that since the proposed imports are propagative material, all the pests will arrive on living hosts. The assessment then discusses – briefly! – the mechanisms by which the pest or pathogen could disperse to infest new trees – e.g., flight, rain splash, irrigation water. However, I think the assessment is sometimes too cautious in describing probable invasive risks.
I also find several important weaknesses in both the risk assessment process generally and specific findings.
Weaknesses of the Risk Assessment Process
The assessors do not discuss the potential efficacy of pest-management actions taken by the exporter or by USDA at ports of entry. They outlined production and harvesting practices that they assumed would apply to the exported plants. They warned that the risk assessment finding could not be applied to plants produced or handled other under conditions.
I am troubled by the assessors’ decision not to consider the plants’ ages and sizes. There is evidence that age and size are very important in determining the likelihood of pest presence. Perhaps the decision reflects the assumption that the exported plants would be less than four years old. Still, the assessors should have been transparent about the reasoning behind this decision.
The assessment underestimates “uncertainty”. One manifestation is the decision to provide little information about whether pests or pathogens known to attack several Eurasian species of Ulmus might also attack North American elm species. This gap arises, I believe, from the International Plant Protection Organization (IPPC) and APHIS requirement that risk analysts consider only pest-host relationships described in the literature or inferred from port interception data. I find this narrow approach to be a weakness, given how many unknown pest-host relationships have proved to be highly damaging. This issue arises specifically in the reviews of the nematode Meloinema kerongense and several powdery mildews (Erysiphe kenjiana, E. ulmi and Podosphaera spiralis) – all of which are identified as affecting at least some elm species.
Perhaps the missing information has fewer consequences here, since the NAPPRA process does not require that APHIS prove the pest-host relationship for every pest evaluated in order to justify retaining the prohibition on importation. The well-documented history of detecting the citrus longhorned beetle in artificially dwarfed trees and as a pest of the Ulmus genus provides more than sufficient justification to retain trade restrictions. Still, if APHIS is conducting a formal risk assessment, it should be thorough. Anything else sets an unfortunate precedent.
Finally, in cases when some of the hosts considered are commercial crops – e.g., fruit trees – the assessment often does not include forest trees as economically important resources at risk.
Questions re: some of specific pests in the analysis
3.2.1. Cerambycidae (Coleoptera)
The risk assessment notes the minimal information available regarding several cerambycid beetles present in Korea that are capable of feeding on elm trees. Collectively, these beetles have a wide host range — Acer, Alnus, Citrus, Ficus, Hibiscus, Juglans, Malus, Morus, Quercus, Populus, Prunus, Pyrus, Salix, Sorbus, and Ulmus. The beetles can thrive in the climate present across most of the Lower 48 states (USDA Plant Hardiness Zones 6-9). The risk assessment does mention the risk to urban and forest trees. It also mentions British detection of A. chinensis larvae in twigs of imported maple trees, but for some reason does not mention past U.S. detections and introductions of this beetle in maple bonsai/bunjae trees in Tukwila, Washington. Is this because the detections were 20 years ago? Does the passage of time make the detections any less relevant?
trees removed for CLB eradication in Tukwila, Washington
The analysis of this tortricid moth notes its broad host range, including Abies, Acer, Betula, Fraxinus, Populus, Quercus, Salix, Sorbus, Tilia, and Ulmus. Yet the analysis makes no mention of the potential impact of moth larval feeding on the buds and flowers of forest trees. Nor does it discuss the moth’s impact in Canada, where it is established. The Canadian experience seems quite pertinent and is an obvious omission.
3.2.3. Meloinema kerongens
This nematode is present on elms in Korea. The assessors could find no information on the damage it causes to its hosts there. Again, there is no discussion of possible vulnerability of American elms. Apparently the nematodes are considered likely to survive the importation process, when the trees will be bare root. The assessors say that since the dwarfed trees (once imported) are likely to be planted in pots, that might limit the nematodes’ dispersal into native soil habitats and ability to infect new trees. This finding is troubling because it is likely that nematodes or their eggs could be present in the pots’ soil, and if that soil leaks from the pot or is disposed of during repotting or with other actions, pests could become established in native soil.
3.2.5. Helicobasidium mompa
This fungus causes root rot on multiple genera in 44 plant families. The list of hosts includes Pinus spp., Populus spp., Prunus spp., and Quercus spp. It appears to thrive in a wide climatic range covering virtually the entire Lower 48 states (USDA Plant Hardiness Zones 2-11). The fungus is spread via rain or irrigation water. I note that experience with the Phytophthora genus of brown algae has demonstrated how difficult it can be to control pathogens that spread in rain or irrigation water – in both nurseries and the wild.
Other Potential Pests
I urge experts to review the long list of pests not analyzed—especially the nematodes that inhabit the root and rhizosphere. Analysts did not analyze them because they are ectoparasites; they decided that ectoparasites were unlikely to remain with the dwarfed trees when they are shipped bare-root.
I also wonder whether the mistletoe Viscum album – a parasitic plant – might be spread onto the dwarfed trees by birds perching on branches or shelter structures above the production facilities. Assessors thought that dormant mistletoe on the plants would not be easily detected during visual inspection at the ports.
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
Arctostaphylos uva-ursi (bear berry); photo by Sten Porse, Commons.Wikipedia
The California Department of Food and Agriculture (CDFA) is seeking input on proposed pest ratings for two species of Phytophthora: Phytophthora occultans and Phytophthora quercetorum. Each has the potential for being a serious pest in California and being spread throughout the United States. Therefore it is important to weigh in on this process. The first deadlines for comments is December 18.
These risk rating proposals can be found at https://blogs.cdfa.ca.gov/Section3162/ The website also has instructions for sending comments. This process can be clumsy so, if it doesn’t work, send your comments directly to the webmaster.
In general, the State assigns each potential pest a rating of A, B, or C. Those rated “A” are most likely to cause harm and also most subject to State regulation. Under “B”, the County Agricultural Commissioners have discretion to take regulatory actions. Pests ranked at “C “are not subject to any State enforced regulatory actions. The “C” rating is supposed to be assigned to pests that are widely distributed in the state and are expected to have a “medium” to “low” impact on vegetation (cultivated or wild) in the state.
Phytophthora occultans
Proposed for risk rank “C “. Comments are due December 18th.
Phytophthora occultans is a recently described species found in nurseries in Europe and in some U.S. states (including Oregon). It was recently detected in the San Francisco area of California.
The State proposes to rate Phytophthora occultans as a level “C” pest. This is insufficient. A rank of “B” is more appropriate, for the following reasons.
1) The data presented in the CDFA proposal are too limited to judge the species’ distribution in California. The proposal refers to only “two detections, two years apart, in San Francisco County.” By ranking it “C”, CDFA seems to assume the pathogen is widespread, based on detections in Europe and other states, without U.S. evidence
The available record does not indicate that CDFA made any attempt to determine the extent of the P.occultans infestations — no survey of other plants at the contractor’s nursery or at other nurseries and no consultation with a larger group of stakeholders.
2) CDFA limits discussion of possible impacts to hosts listed in the literature –which belong to multiple plant families. It makes no mention that additional hosts are likely to be discovered (as has often happened with regard to the host ranges of other pathogens in the Phytophthora genus). If the host range expands, as I expect it will, the impact to restoration activities, rare plants, wildlands and nurseries is more likely to be significant, not medium to low.
Furthermore, several of the known host species are congeners of species that are federally listed as endangered or threatened, i.e., species in the genera Ceanothus and Arctostaphylos. I think it is highly unwise to disregard in risk assessments the probability that listed species will prove to be hosts.
In conclusion, please submit comments to California Department of Food and Agriculture urging it to assign a risk rating of “B” to Phytophthora occultans.
Instructions are contained in the proposal. If this process doesn’t work (sometimes it is clumsy), send your comments directly to the webmaster.
Over the past 5 years, P. quercetorum has been detected in association with oak trees, primarily coast live oak (Quercus agrifolia), in four counties in California, two in the Central Valley (Fresno, Sacramento), two on either side of the San Francisco Bay (Alameda, San Francisco). There have been no interceptions of the species by CDFA border inspectors. The species had earlier been associated with oak roots and rhizosphere soil of oak forests in the eastern and north central US. Its pathogenicity is said to be unknown – and difficult to separate from impacts of other, often co-occuring Phytophthoras. CDFA assigns a rank of “high” with regard to economic impact, although it says there are no reports quantifying economic losses in plant production facilities.
CDFA believes that the species is likely to be able to establish wherever its hosts can grow (a rank of “high”). Hosts include red maple (Acer rubrum), English ivy (Hedera helix), several eastern oaks, and a second California oak, interior live oak (Q. wislizeni). CDFA assigns this a rank of “moderate” host range.
The environmental impact is ranked as “high” since the pest could lower biodiversity, disrupt natural communities, or change ecosystem processes; and the pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.
The overall ranking for the “Consequences of Introduction” is “high”.
However, the recommended ranking is “C”, which – again – means the pest is not subject to any State enforced regulatory actions. “C” rated pests are widely distributed in the state and are expected to have a “medium” to “low” impact on vegetation (cultivated or wild) in the state.
Why would CDFA recommend “no action” for yet another Phytophthora species that is known to attack two of the state’s most ecologically important oaks and possibly many more species? Even when the exact impacts are unclear … Especially when the principal means of spread is planting trees in restoration areas – a deliberate human action.
According to the USDA Forest Service, coast live oak (Quercus agrifolia) is a conspicuous tree in lower-elevation oak woodlands of California, which collectively occupy about 10 million. It is co-dominant in the southern oak woodlands. CLO trees generally occur on mesic sites such as north slopes, alluvial terraces, canyon bottoms, or upper streambanks. Coast live oak woodlands are some of the most important habitats to wildlife in California; they provide habitat for black bear, black-tailed deer, rodents and lagomorphs, and various upland game and nongame birds – including those that feed on acorns and cavity nesters. The birds including the federally endangered least Bell’s vireo and least tern.
Coast live oak is more fire resistant than other California oak species.
Coast live oak is favored for use in rehabilitation projects throughout its range. It is used in watershed improvement, restoration, and wildlife habitat rehabilitation projects.
CLO is already under pressure by predation by deer and cattle; sudden oak death (SOD; causal agent Phytophthora ramorum); goldspotted oak borer (GSOB – Agrilus auroguttatus); and sometimes the polyphagous shot hole borer (PSHB; Euwallaceawhitfordiodendrus) and its associated Fusarium fungus. [These three non-native organisms are described here.]
range of Q. wislizeni; USDA Forest Service map
According to the USDA Forest Service, interior live oak (Quercus wislizeni) occurs over about 16% of California’s landscape, especially in the Inner Coast Ranges, the foothills of the southern Cascade Range, and the Sierra Nevada. Among California’s red oaks, interior live oak has the highest tolerance for xeric conditions. It usually dominates the “scrub” or “live oak” chaparral vegetation types in the Inner Coast Ranges and the Sierra Nevada.
CNPS Calscape lists several insects associated with the species.
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
As we know, the SOD pathogen Phytophthora ramorum infects more than 100 plant species [APHIS host list posted here]. Some are killed, some not. Some support production of spores (=sporulation), and thus promote spread of the disease – either in nurseries and plantings, or in the wild. Conditions under which P. ramorum infects specific plant species also varies.
In both the ornamental plant industry and natural environments, transmission is driven mostly by foliar hosts.
Matteo Garbelotto and colleagues have carried out studies aimed at improving our understanding of the differences in host-pathogen interactions, and their meaning vis a vis persistence and spread of the disease – especially in wildland situations. The experiments were carried out five or more years ago, funded by the Farm bill Section 7721 funding. See the full reference at the end of this blog.
The team ranked 25 ornamental plant species representing ten families for susceptibility to P. ramorum and infectivity (spore production). They also tested potential differences among three of the genetic lineages of the pathogen—NA1 (prevalent in U.S. forests), NA2 (found in some nurseries in Pacific coast states), and EU1 (found in nurseries and – since 2015 – in some wildland forests in Oregon). The team also studied the effect of temperature on infectibility. Their goal was to help focus regulations so they will be more effective.
The studies clearly show that the relationship between P. ramorum and various hosts is complex – both susceptibility and infectibility vary depending on the host species, pathogen genetic lineage, and environmental conditions, especially temperature. Results of testing of leaves for the presence of the pathogen were affected by such experimental choices as the concentration of zoospores, temperature, plant host, pathogen genotype, and by the interaction between host and pathogen genotype. Stem results were mostly affected by host and host-pathogen genotype interaction.
Hosts bearing the most severe infections do not always support the highest levels of sporulation, so they are not necessarily the most likely to spread the disease.
Regulators also cannot always generalize re: the pathogen’s impact on plant hosts based on the hosts’ taxonomic relationship. Results were fairly similar for congeneric species within the genera Rosa, Prunus, and Syringa, but quite different for species within the genera Ilex, Gaultheria, and Osmanthus.
It is clear that basing regulatory or best management practices on any one pathogen-host-environment relationship is likely to lead to failure, leaving our forests inadequately protected
The findings that pertain most directly to early detection of infections and those that otherwise promote spread of the pathogen are my focus here.
Hosts that Support Sporulation / Spread of Disease
At least five host species are much more infectious than Rhododendron catawbiense. Hosts that support the highest levels of sporulation were Syringa vulgaris, Hamamelis intermedia, and Syringa meyeri. Hosts that support medium-high levels of sporulation were Rosa gymnocarpa and Syringa pubescens subsp. patula.
Two of the Syringa species support high levels of sporulation, but rank low on overall susceptibility. Rosa gymnocarpa ranked fourth for levels of sporulation, but only fifteenth for overall susceptibility. At least six other species join this group of taxa that are highly infectious without displaying noticeable symptoms. Note than none of these top disease drivers is included in the so-called “filthy five” genera which are the focus of federal and state detection efforts. These genera are Rhododendron spp., Camellia spp., Viburnum spp., Pieris spp., and Kalmia spp.
One of the “filthy five” is Rhododendron catawbiense. It is often used as a standard against which to compare other species’ vulnerability. R. catawbiense supports a somewhat lower level of sporulation than do the species listed in the preceding paragraph. Again, disease severity is not a reliable cue to the likelihood of supporting sporulation and disease spread. Thus, the Hamamelis intermedia was the only species that scored high for both sporulation and susceptibility.
Temperatures Affect Infection Rates
A temperature of 20°C [68o F] was found to be ideal for maximum sporulation by all three genotypes. However, the NA1 genotype was a relatively good sporulator at 12oC [53oF]. The NA2 genotype sporulates prolifically at 25°C [77oF], but produces fewer sporangia than the other two genotypes at 12oC. These findings suggest which genotype might pose a greater risk in warmer or cooler regions than those supporting the current wildland infestations in California and Oregon. Thus, if NA2 spreads via the nursery trade to warmer regions, such as the area of the Southeast identified by various risk maps developed in the past [See maps on pages 14 – 16 in chapter 5 of Fading forests III, available here], it might pose a higher risk. This discovery intensifies concern arising from the fact that many of the P. ramorum-infected plants shipped to Indiana – and presumably other eastern states – in 2019 were of the NA2 lineage. States that received infected plants in 2019 included Alabama, Arkansas, Kentucky, Missouri, North Carolina, Tennessee, Virginia, and West Virginia.
Considering individual host species, Gaultheria shallon, R. catawbiense, Osmathus delayayi and Hamamelis intermedia supported good sporulation at the higher temperatures whereas Laurus nobilis, Syringa vulgaris, and Magnolia stellata supported better sporulation in cooler climates. Note that H. intermedia and S. vulgaris support prolific sporulation; the latter is a “symptomless superspreader”.
Garbelotto et al. note that Magnolia stellata is both highly susceptible and highly infectious at 12°C and thus able to spread the infection in colder areas. This advice to limit use of this species in cooler areas runs counter to horticultural experts’ guidance to plant this shrub in USDA Hardiness Zones 4–9 – which include virtually all the lower 48 except the most northern parts of Montana, North Dakota, and Minnesota. Clearly, star magnolia is a popular plant in colder regions. At the other end of the spectrum, Gaultheria shallon, Hamamelis intermedia, and Mahonia aquifolia were both highly susceptible and infectious at 25 °C, thus their use should be limited in warmer areas. All three include warm regions in their native ranges.
Early Detection
There are two ways to carry out early detection surveys.
(1) The first is detection of infection in plants themselves. Garbelotto et al. determined that 14 plant species are highly or moderately susceptible to infection even with relatively limited inoculum sources. Intense monitoring of these species would be likely to detect new infestations. Three of the highly susceptiblespecies, namely Syringa meyeri, Syringa pubescens subsp. patula and Hamamelis intermedia, are potentially more susceptible than R. catawbiense.
Hamamelis x intermedia ‘Angelly’ 01.JPG Wikimedia Commons
Based on the relative ease of pathogen re-isolation from the following host species after they had been inoculated at low levels, Syringa meyeri, Syringa pubescens subsp. patula, Hamamelis intermedia, Syringa vulgaris, Osmanthus delavayi, and Magnolia grandiflora indicated that a larger number of plants in the production facility had become infected.
(2) A second approach to early detection monitoring would be to focus on those host taxa able to support the most robust sporulation when infected by low levels of inoculum. This approach emphasizes curtailing spread.
As I noted above, Garbelotto et al. conclude that five species could spur significantly faster disease spread due to higher transmission rates coupled with higher susceptibility rates. These five species are Syringa vulgaris, S. meyeri, and S. pubescens subsp. patula; Hamamelis intermedia; and Rosa gymnocarpa. Note than none of these disease drivers is included in the so-called “filthy five” genera on which regulators focus now detection efforts.
Several species appeared less diseased, but supported more vigorous sporulation (e.g., Syringa vulgaris,S. pubescens subsp. patula and Rosa gymnocarpa). Others were more diseased but supported less sporulation (e.g., Prunus laurocerasus and Prunus lusitanica). Therefore, nursery managers and regulators should not rely on visual assessment of disease intensity to judge spread risk.
Other Information
Comparing the three genotypes, EU1 was most aggressive in terms of disease incidence at both low and high inoculum loads. At low levels of inoculum, NA1 lineage was comparable in terms of disease severity.
However, at higher inoculum loads NA1 was clearly the most infectious based on the number of sporangia produced on infected hosts. Garbelotto et al. conclude that the co-mingling of the EU1 and NA1 lineages in Oregon forests might result in a highly destructive forest disease, as both virulence and transmission potential would be maximized. There is the further risk that the presence of the two genetic lineages, which have different mating types, might enable sexual reproduction/ genetic exchange between the two lineages.
Sources
Matteo Garbelotto, M., D. Schmidt, T. Popenuck. 2020. Pathogenicity and infectivity of Phytophthora ramorum vary depending on host species, infected plant part, inoculum potential, pathogen genotype, and temperature. Plant Pathology 2020;00.1
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
