Oak wilt response framework for Canada
On this page
- 1. Foreword
- 2. Introduction
- 3. Objectives
- 4. Partners and stakeholders
- 5. Outreach and education
- 6. Significance and implications
- 7. Host
- 8. Means of dispersal
- 9. Regulatory tools
- 10. Response framework
- 11. Research
An Oak Wilt Technical Advisory Committee (OWTAC) has been established to coordinate ongoing multi-government information sharing and actions in support of oak wilt preparedness and response in Canada. The team of technical experts from a number of federal, provincial, municipal and non-profit organizations strives to facilitate and support research advancements, recommend outreach, education, and communication strategies to increase oak wilt awareness. Members of the committee also liaise with experts from other science or advisory bodies. The Oak Wilt Technical Advisory Committee worked collaboratively to produce the following framework, outlining comprehensive measures that may be implemented in the event of an oak wilt incursion in Canada. Representatives from the following organizations contributed to the framework in an effort to mitigate risks and protect the oak resource.
- Canadian Food Inspection Agency
- Ontario Ministry of Natural Resources and Forestry
- Natural Resources Canada – Canadian Forest Service
- Ontario Ministry of Agriculture, Food and Rural Affairs
- Invasive Species Centre
- Ministère des Forêts, de la Faune et des Parcs du Québec
- City of Windsor
- New York Department of Environment and Conservation
- Michigan Department of Natural Resources
Oak wilt is a vascular wilt disease caused by the fungus Bretziella fagacearum (previously known as Ceratocystis fagacearum). The fungusdevelops on the outer sapwood of the tree, causing its host to develop tyloses and gums which block the movement of water and nutrients within the xylem, resulting in branch wilting and tree death. The disease is capable of killing trees in a single season. Oak wilt is primarily spread from diseased to healthy trees through root grafting. This disease can also be spread by oak bark beetles, which transmit spores while creating galleries within infected tissue, and by sap beetles, which transmit spores while feeding on fungus mats. Species of red oak are infected more frequently and die much more quickly than white oaks. Although thousands of trees have died in the United States (U.S.) as a result of oak wilt, mortality rates are higher in the Midwest than in the southeast range of this disease.
Oak wilt was first recognized as an important forest pest in 1944 in Wisconsin and is now known to occur in 24 states within the U.S. (South Dakota to western New York, southward to South Carolina, westward to Texas and northward through Oklahoma to Nebraska). This disease is not known to occur in Canada, however, in 2016, it was detected in Michigan less than 1 km from Canada.
The Oak Wilt Response Framework outlines comprehensive measures that may be implemented to address oak wilt incursions in Canada. The document is intended to foster ongoing dialogue and information sharing under a collaborative framework.
4. Partners and stakeholders
Partnerships with a number of organizations help support oak protection and conservation efforts. The following collaborators may play a role in effectively managing incursions of invasive species like oak wilt in Canada:
- Natural Resources Canada – Canadian Forest Service
- Parks Canada
- Provinces and territories (for example, natural resources, agriculture, and environment)
- Private woodlot owners
- First Nations
- Invasive Species Councils
- Non-government organizations
- United States Department of Agriculture (USDA)
- United States departments of natural resources
5. Outreach and education
Outreach and education prior to an oak wilt incursion is critical because it facilitates stakeholder and community awareness and support, which in turn allows for the possibility of rapid action to be taken when required. Community and stakeholder awareness has already begun with pest information on oak wilt being provided and distributed through various channels, including pest factsheets (CFIA 2016 and Invasive Species Centre 2017) and paid social media campaigns on Twitter and Facebook. Oak wilt training sessions are ongoing with representatives from municipalities, conservation authorities and parks, government, First Nations, Non-governmental organizations and associations, as well as arborists and tree care professionals considered the target audience. Materials and handouts from the sessions are available on the Forest Invasives' oak wilt page. Additional sessions will be considered in the future based on ongoing consultation with partners to ensure educational needs are addressed.
6. Significance and implications
6.1 Trade impacts
Oak lumber exports from Canada were worth $74M in 2015. Exports have increased in recent years. From 2013 to 2015, China, the United States and the United Kingdom were the top 3 countries importing Canadian oak lumber (Statictics Canada 2017). Importation from the U.S. to Canada of oak seedlings/plants in 2015-2016 was mostly to British Columbia and Ontario.
6.2 Ecological impacts
Oak plays a vital ecological role in eastern Canada's urban and natural forests. Loss of oak has the potential to reduce biodiversity and to cause a reduction in habitat and food for wildlife with acorns considered the most important wildlife food in deciduous forests of North America. Oak wilt may also result in potentially significant losses in environmental services as oak trees help to stabilize slopes, limit soil erosion and reduce air pollution.
Oak wilt is known to attack fagaceous hosts. All species of oak present in the U.S. have been demonstrated to be susceptible to oak wilt; however, their susceptibility varies greatly according to their taxonomic group (Chalkley 2016). Members of the red oak group (Quercus section Lobatae) (Table 1) are known to be highly susceptible to the fungus and usually die shortly after infection (Wilson 2005). Members of the white oak group (Quercus section Quercus) (Table 2) have been known to have moderate to high level of resistance (Juzwik et al. 2011). White oak species generally experience moderate dieback which can ultimately lead to tree death after several years (Wilson 2005). On the contrary, a study has reported that European oak of the white oak group planted in the U.S. (for example, Q. robur) were highly susceptible to the fungus (MacDonald et al. 2001).
A few non-Quercus tree species were also reported to be susceptible to B. fagacearum, including four Castanea sp. (C. dentata, C. mollissima, C. pumila and C. sativa), Notholithocarpus densiflorus (syn. Lithocarpus densiflorus), Chrysolepis sempervirens (syn. Castanopsis sempervirens) and Malus domestica (Bart 1960; Bretz 1951; Bretz and Long 1950; Bretz 1952; Bretz 1955; Bretz 1957; Ernst and Bretz 1953; Manos et al. 2008; Merrill 1975; USDA NRCS 2018). With the exception of C. mollissima, all reports for non-Quercus spp. are the result of artificial inoculation of the fungus. There is currently a lack of evidence suggesting that these species should be considered natural hosts.
|Scientific name||Common name||Distribution|
|Quercus coccinea||Scarlet oak||Uncommon. Native to the eastern United States (U.S.)|
|Q. ellipsoidalis||Northern pin oak||Uncommon. Occurs in Ontario (ON) west of Lake Superior and north of Lake Erie.|
|Q. ilicifolia||Bear oak||Uncommon. Occurs in Southern ON.|
|Q. palustris||Pin oak||Common. Occurs in southern ON north of Lake Erie and Lake Ontario.|
|Q. rubra||Red oak||Common in eastern Canada, east of Lake Superior to Nova Scotia (NS).|
|Q. shumardii||Shumard oak||Uncommon. Occurs in southern ON. Population evaluated to less than 1000 trees in 40 locations in 5 counties Table Note 2.|
|Q. velutina||Black oak||Occurs in ON, north of Lake Erie and Lake Ontario.|
|Scientific name||Common name||Distribution|
|Quercus alba||White oak||Common in southern ON and southern Quebec (QC)|
|Q. bicolor||Swamp white oak||Occurs in southern ON and southern QC|
|Q. garryana||Garry oak||Uncommon. Scattered in British Columbia (BC) (Vancouver Island, Gulf Island, lower Fraser River)|
|Q. macrocarpa||Bur oak||Most common white oak. Found southern Saskatchewan (SK) east to NB.|
|Q. muehlenbergii||Chinquapin oak||Uncommon. Occurs in restricted area of southern ON|
|Q. prinoides||Dwarf chinquapin oak||Uncommon. Occurs in southern ON in restricted areas.|
|Q. robur||English oak||Naturalized locally. Found in BC, ON, NB, NS and Prince-Edwards Island.|
- Table Note 3
Brouillet et al. (2010+); Farrar (1995); Flora of North America Editorial Committee (1993+); Gucker (2006)
8. Means of dispersal
First detected in the 1940's in the Upper Mississippi Valley, oak wilt was later found to be well established in eastern and mid-western U.S. following surveys. Since then, B. fagacearum increase in distribution has been slow (Juzwik et al. 2008). New detections occasionally occur in counties or states outside the known range of distribution, however, it is believed that the lack of new reports in areas where it was previously reported suggest that the fungus may no longer be present in some areas (Juzwik et al. 2011; Michigan Department of Natural Resources 2015). Hence, this indicates that B. fagacearum overall spread rate is quite slow.
Spread of B. fagacearum has been reported to occur through natural means belowground by root grafting and aboveground by insects. Human-mediated movement of infected material has been found to be responsible for spreading the fungus to new areas over longer distances (Koch et al. 2010).
8.1 Natural spread
8.1.1 Root grafting
Belowground spread of B. fagacearum occurs through root grafting. Once introduced in a new area, root grafting is usually the most important means of local spread (CABI and European Mediterranean Plant Protection Organisation 1997). In Minnesota, where the disease has been present for several decades, over 95% of diseased trees have been reported to be infected through root graft (Bruhn and Heyd 1992). However, the importance of root grafting as a mechanism to spread the disease depends on a number of environmental factors that affect its frequency, for example, the oak species present, forest composition, the size of the trees, soil type and terrain (Juzwik et al. 2011; Juzwik et al. 2015). All these factors influence the rate of spread within an infection centre which will, therefore, vary greatly from one location to another.
Root grafting occurs most frequently between Quercus trees of the same species. Forests composed of many species have been reported to have a reduced rate of spread as opposed to pure stands containing only one single oak species (Wilson 2005). The probability of root grafting is also dependant on the distance of healthy trees from the diseased tree. Sandy soil will promote root grafting, while rocky soils and steep slopes have been suggested to negatively impact root graft formation (Juzwik et al. 2011). In Minnesota, a study reported that most of the spread through root grafts occurred within 9.1 m of infected trees but could be as high as 15.2 m depending on environmental factors (Juzwik et al. 2015). Overall, disease spread by root grafting occurs over short distance. For example, in Michigan and Minnesota, two states bordering Canada, disease spread within a specific area through root grafting occurred at a rate between 1.9 to 12 m/year (Juzwik 2007). Special soil conditions such as those observed underneath roads can greatly decrease the risk of root graft formation and disease dispersal. It is thought that oak roots do not grow under paved roads or hard packed gravel roads (Bruhn and Heyd 1992).
Aboveground spread is generally responsible for the establishment of new infection centers in nearby or distant forest stands (Juzwik 2000). The aboveground dissemination of B. fagacearum is caused by the dispersal of spores from the fungal mat. Fungal mat formation is a unique feature of this pathogen. When infected red oak trees die, spore mats, which arise from the fungus mycelium growing from the infected xylem, are produced on the trunk and on large branches. Fungal mats can eventually evolve to form pressure pads which will cause the bark to split. The fruity smell produced by fungal mats can be sensed several meters away from infected trees and attracts a number of insect species (Juzwik et al. 2011). When insects visit the fungal mats, conidia and ascospores stick to the insect body. Studies have demonstrated that a number of insect species can vector B. fagacearum (Juzwik et al. 2011).
Fungal mats have been reported from trees in the red oak group. Trees in the white oak group have very rarely been reported to develop fungal mats and generally, are not considered a good source of inoculum for further spread of the disease into new areas (Wilson 2005). Few reports of fungal mats on species of the white oak group were from artificial inoculation studies which are not representative of natural conditions (Engelhard 1955; Nair and Kuntz 1963). To our knowledge, there is only one report of fungal mat and pressure pad production on a Q. alba in a natural setting (Cones 1967).
Formation of fungal mats, their longevity, and abundance is influenced by environmental conditions (i.e. moisture and temperature). Fungal mats usually form within a year of tree death and are generally found on trees bigger than 15 cm in diameter at breast height (O'Brien et al. 2011). They are produced in the spring and fall when environmental conditions are conducive to their formation (Juzwik et al. 2015; Koch et al. 2010). Fungal mats have been reported on branches as small as 2-4 cm in diameter but they occur less frequently on smaller branches and this may be due to the more rapid drying of the sapwood in comparison to larger branches (Gibbs and French 1980). Their production also depends on the time of infection and death of the tree (Chalkley 2016). Fungal mats produce spores for two to three weeks and during that period; they serve as inoculum to spread the fungus to healthy hosts.
According to several research studies on this pathogen, two insect families are regarded as the main vectors responsible for the dissemination of spores of B. fagacearum i.e. sap beetles (nitidulids) and oak bark beetles (scolytids). It is important to note that, although, they are reported to be the main vectors, these insects are still considered to be inefficient vectors (Wilson 2005). Evidence suggests that sap beetles Carpophilus spp. and Colopterus spp. are the main genera involved in the spread of the fungus (Juzwik et al. 2011). Colopterus truncatus plays a key role in dissemination throughout the range of oak wilt (Hayslett et al. 2008). Other nitidulid vectors appear specific to different regions. Carpophilus sayi is considered an important vector in the upper Midwest and Colopterus semitectus and Colopterus niger are vectors in Missouri(Hayslett et al. 2008; Juzwik et al. 2004). Colopterus truncatus has been reported from all Canadian provinces except Prince Edward Island and Newfoundland and Labrador (Bousquet et al. 2013). Carpophilus sayi has been reported from Saskatchewan, Manitoba, Quebec, New Brunswick and Nova Scotia (Bousquet et al. 2013). Species of Epuraea and Glischrochilus have also been reported to be vectors (Cease and Juzwik 2001; Hayslett et al. 2008).
The sap beetles are attracted by the fruity smell of spore mats and volatile chemicals released by wounded trees. Sap beetles feeding or ovipositing on fungal mats of infected trees can then carry fungal spores to fresh wounds of healthy trees and establish new infection centres (Juzwik et al. 2004). Wounds less than 72 hours old are usually required for disease transmission (Juzwik et al. 2011). These beetles are most active in spring and early summer (April to mid-July in Minnesota) (Juzwik et al. 2015). Timing of fungal mat formation and degeneration, availability of wounds and the timing of the emergence of the vector are factors that will affect the efficiency of disease transmission (Gibbs and French 1980).
Studies have reported that oak bark beetles Pseudopityophthorus minutissimus and P. pruinosus can also contribute to disease spread; however, to a lesser extent than sap beetles (Ambourn et al. 2006; Juzwik et al. 2015). The mechanism by which bark beetles transmit the fungus is not well understood. It is believed that they may be contaminated as the larvae bore galleries in infected trees before adult emergence. During their maturation and reproductive activity, the beetle creates wounds that allow the fungus to infect new trees (Juzwik et al. 2011). The reduced survival of B. fagacearum in small branches where oak bark beetles generally breed may explain the lower importance of these vectors in disease transmission (Webber 2015). In Canada, while P. minutissimus has been reported from Ontario, Quebec, New Brunswick and Nova Scotia, P. pruinosus is not known to be present.
8.1.3 Other Means of Dispersal
Some researchers currently believe that there are unknown factors involved in the dispersal of oak wilt but their role in disease spread is unclear (Wilson 2005). Weather events such as storms could increase the disease incidence and spread of this fungus. For example, in July 2011 in Minnesota, many trees in an uninfested area were damaged by a large storm prior to the detection of many oak wilt infested trees in 2012. Since the vectors usually only travel short distances, it is believed that the pathogen may have already been present at low levels in the area and took advantage of the numerous wounds caused by the storms (Michigan Department of Natural Resources 2012). Alternatively, we could hypothesize that contaminated vectors or spores may be carried to new locations by strong storm events. Migratory birds, especially sapsuckers that feed on inner bark and cambium tissues, have also been suggested as a potential pathway for the long distance spread of B. fagacearum (Juzwik et al. 2008). Squirrels have previously been reported to feed on fungal mats and to carry the fungus under artificial conditions but there is little reason to believe that this is actually happening in nature (Gibbs and French 1980; Juzwik et al. 2011).
8.2 Human-assisted spread
8.2.1 Logs, firewood and other plant material
The movement of firewood and logs can disseminate B. fagacearum over long distances. Logs and firewood originating from infested trees may harbor fungal mats that can serve to introduce the pathogen into new areas via insect vectors. Transport of infected firewood from infested places (for example, cottage, camping site, etc.) poses an important risk of introducing this pathogen to new areas (Michigan Department of Natural Resources 2015). Some evidence suggests that the introduction in New York (2008), Texas and further spread in Michigan may have been caused by the movement of infected firewood (Jensen-Tracy et al. 2009; Juzwik 2007; Juzwik et al. 2011; Juzwik et al. 2008). Similarly, the transport of infected logs with fungal mats would be a possible pathway for the introduction of B. fagacearum to new continents (CABI and European Mediterranean Plant Protection Organisation 1997; Chalkley 2016). Oak bark beetles could be present in wood with bark and serve as a pathway for introduction into new areas (CABI and European Mediterranean Plant Protection Organisation 1997).
There is no evidence to suggest that this disease is transmitted by acorn stock (Juzwik et al. 2011). Nursery plants could potentially be infected by the pathogen; however, there is a lack of evidence suggesting that they could serve as a potential pathway for the spread of this pathogen (Juzwik et al. 2011; Roberts 2016a). There has been no report of a natural oak wilt infection on bare root or potted oak trees (Juzwik et al. 2008). Oak wilt has commonly been reported from forest trees but not from nursery plants (CABI and European Mediterranean Plant Protection Organisation 1997). Speculatively, if nursery plants, especially from the red oak group, would be infected in the spring (May to July) by insect vectors, they would be expected to exhibit severe symptoms, rapidly decline and would not be saleable or exportable the following spring.
8.2.2 Human activity
According to the Michigan Department of Natural Resources (2015), in urban areas, most new oak wilt outbreaks can be traced back to damage from pruning, construction and other tree-wounding activities that occurred in the spring. In forests, activities that may wound trees in the spring, such as harvesting, thinning and road construction, can lead to the spread of the pathogen to new areas (Juzwik 2007).
9. Regulatory tools
Federal or Provincial legislation may be used to regulate the movement of all propagative and non-propagative commodities of Quercus spp. (Oak) to control the entry and spread of oak wilt. The Invasive Species Act is a piece of legislation unique to Ontario that allows for action to be taken regardless of the ownership (i.e. private, crown, public) of the land where a monitored invasive species is found (Government of Ontario 2015). This type of legislation may prove useful for oak wilt management and should be considered. Oak wilt is a pest which is regulated by the CFIA under the federal Plant Protection Act and Regulations. The regulatory instruments and documents mentioned below are some of the tools that can be used to control oak wilt.
9.1 Import and domestic requirements
The CFIA Directive D-99-03 Phytosanitary Measures to Prevent the Entry of Oak Wilt Disease (Ceratocystis fagacearum (Bretz) Hunt) from the Continental United States identifies the specific areas in the United States (U.S.) where oak wilt is considered to be established and outlines the import requirements for oak wilt hosts, including propagative and non-propagative material.
The CFIA directive D-01-12: Phytosanitary Requirements for the Importation and Domestic Movement of Firewood contains the phytosanitary requirements for the import of firewood to prevent the entry to Canada of quarantine pests from all areas of the world, including Canada.
Information concerning regulated commodities can be obtained from the Automated Import Reference System (AIRS) website.
9.2 Reporting and notification of a regulated pest
Under Section 5 of the federal Plant Protection Act, when a person becomes aware of the existence of a plant pest in an area where the pest has not been previously known to exist, the person must immediately notify the CFIA and provide a specimen where possible.
Under the International Plant Protection Convention (1997), Canada is required to report the occurrence, outbreak and spread of quarantine pests (International Plant Protection Convention 2002). CFIA has the responsibility to carry out pest surveys and to verify all pest records. Pest reports containing information on the identity of the pest, location, pest status, and nature of the immediate or potential danger, including changes to pest status must be communicated to other countries, particularly to the U.S. and other trading partners.
9.3 Identification of a regulated area
A regulated area may be established to control the entry and spread of a regulated pest under the authority of the federal Plant Protection Act. Initially, a regulated area would be defined through the issuance of a Notice of Prohibition of Movement (PoM). Depending on the spread and other considerations a Ministerial Order (MO) pursuant to subsection 15(3) under the authority of the Plant Protection Act may be issued, or the pest may be added to Schedule II (2) of the Plant Protection Regulations. A Directive (D-Memo) may be published with a map or plan pursuant to section 16 of the Plant Protection Regulations. After the pest has been added to Schedule II (2) the MO is repealed.
9.4 Notice of prohibition of movement
Notices of Prohibition of Movement are issued by a CFIA inspector, pursuant to section 22 of the Plant Protection Regulations, to persons in care or control of regulated materials or properties and places prohibitions or restrictions on the movement of regulated materials from a property. These notices may be issued specific to a property or premises and are used if there is evidence that a pest of quarantine significance is present, or if there is reason to believe it may be present (i.e. an adjacent property to a property where trees have been found infested along the border, where there is no physical boundary or road), and it is necessary to prevent the spread of this pest from the property. There is no date of expiry for the notices and they are valid until revoked in writing by an inspector.
9.5 Movement certificate
A Movement Certificate is signed by an inspector and is issued pursuant to the federal Plant Protection Act, S.C. 1990, c. 22 to authorize the movement of regulated articles within Canada. Only an inspector may authorize, in writing, the movement of the pest, any regulated article, or any conveyance that carries or has carried the pest or any regulated article, or any regulated article originating inside of the regulated area to move out of the regulated area. No person shall move the pest or any regulated article through the regulated area, unless it is in a container that is sealed or closed in a manner that prevents the escape from, access to, removal, or release of the pest or the regulated article and is conveyed through the regulated area without stopping. This is permitted if authorized in writing by an inspector in accordance with a Movement Certificate.
Anyone found violating the restrictions imposed by the above noted federal regulatory instruments, Ministerial Orders, or the Plant Protection Act and Regulations is subject to a fine or prosecution either under the Plant Protection Act and Regulations or the Agriculture and Agri-Food Administrative Monetary Penalties Act.
10. Response framework
A Communications Strategy will be developed to support the Oak Wilt Response Framework. Public cooperation in detecting, and reporting oak wilt and observing movement restrictions to control its spread are paramount to help control the spread of the disease. Communication objectives will be to raise public awareness and education about oak wilt prior to, and following detection in Canada. Prior to detection, the objectives will be to raise public awareness and education about the disease; build awareness about the dangers of moving firewood and wounding trees during high risk periods; and inform the public that CFIA inspectors will be conducting surveys. Post-detection, the goal will be to educate and inform the public about movement restrictions and to encourage reporting of any suspected finds of oak wilt to the CFIA.
The Communications approach will incorporate a mix of communications vehicles including digital media; collaborating with other federal, provincial and municipal levels of government on joint publications; and engagement with industry stakeholders to identify priorities.
Communications products will include news releases; media lines; industry bulletins; maps; and digital content to promote public awareness.
The strategy will be regularly reviewed and updated as required.
10.2 Detection and monitoring
10.2.1 Detection methods
Early detection of oak wilt is critical to rapid response and control of oak wilt in Canada. To support early detection in areas where oak wilt is not known to occur, survey priorities are established based on an analysis of the oak resource in association with risk factors contributing to oak wilt introductions. Visual detection surveys are delivered in accordance with the CFIA Oak Wilt Survey Protocol, available on the Forest Invasives Oak Wilt page or upon request.
Symptoms of oak wilt may vary among oak species and can be confused with oak decline caused by other factors. Suspect reports can be made to by email to firstname.lastname@example.org or by contacting the local CFIA office. Official confirmation of suspected oak wilt incursions must be completed at the CFIA Plant Pathology Laboratory for regulatory purposes. Branch samples from suspect trees are triaged by the CFIA Plant Pathology Laboratory and examined for symptomatic tissue. If found, sectioned longitudinally the symptomatic tissue is removed, ground in liquid nitrogen and sent for DNA extraction-qPCR and also plated for culture to test for the oak wilt fungus.
10.2.3 Delimitation and monitoring
Once oak wilt has been confirmed in an area, the extent of the infestation will be assessed by completing an intensive visual survey around the infested tree(s) in accordance with the delimitation survey procedures outlined in section 6.2 of the CFIA Oak Wilt Survey Protocol. When control measures are implemented at an infested site, site monitoring should be performed in subsequent years in order to detect any resurgence of the disease. If active control measures are applied, 5 years of negative survey data would be required to declare eradication.
10.3 Control options
Several management options that can limit the spread of B. fagacearum are available. The effectiveness of these measures is dependent on a number of key factors including environmental conditions, stand diversity and timing of detection. Once an infection center is identified, case-by-case analysis will need to be undertaken and a site-specific plan will be developed to align with the control objectives. Management measures should attempt to prevent spread through root grafting and limit fungal mat production on recently killed trees (Wysong and Sharon 1986). Measures considered should include regulatory control, delineation of the infested area, root disruption, sanitation and site monitoring. Chemical treatment may be used as a supplementary management tool in some cases. Good management practices have been reported to successfully control the spread of the fungus. For example, in Michigan, increased monitoring and management of infection centres have been successful in eliminating the disease from those areas, with a few exceptions (Juzwik et al. 2011; Michigan Department of Natural Resources 2015). Measures have also resulted in the reduction of the size of the epicenters.
10.3.1 Root disruption
The spread of oak wilt through root grafting can be reduced significantly through the severing of root connections between oak trees. Root grafts between the tree within and outside the infested area are usually disrupted by building a trench.
In preparation for trenching, the following steps should be followed:
- identify symptomatic tree
- measure the diameter at breast height (dbh) of the symptomatic oak on the edge of the infested area and of the healthy trees beyond the infested area
- determine the minimal distance required between symptomatic and healthy tree based on Table 3
- delimit the edge of the infested area / trench line (Figure 1)
total Table Note 6
- Table Note 4
Tree diameter at a height of 135 cm
- Table Note 5
Distance between infected and healthy tree to have at least 95% chance for the healthy tree to survive one more year.
- Table Note 6
The sum of the dbh of the infected and healthy tree
Source: Bruhn and Heyd (1992)
Figure 1: Example of trench lines around an infested area. Based on Table 3, Tree A should be 13.6 m (34 feet) away from an infected tree in a loamy sand and 17.1 m (42.7 feet) in a sandy soil to conclude that the tree is not infected (95% confidence level). Since tree A is farther than the recommended guidelines for loamy sand, a barrier can be placed between the healthy and diseased tree. On sandy soil, tree A is less than the recommended safe distance, so one can conclude with 95% certainty that the tree has been infected and that a barrier must be placed outside it.
Source: Carlson et al. (2010)
Description for Form 1: Product Description
In urban areas, trenching may present challenges due to the presence of many obstacles. Some of these obstacles may be efficient in limiting the spread of the disease. For example, roads are considered to be a significant barrier to oak root growth, root grafts and consequently, belowground disease spread (Bruhn and Heyd 1992). The trenching line should be adjusted to take into account road and other obstacles that could limit root growth.
Once the trench line location has been established, various equipment can be used to dig the trench including a vibration plow, belt trenchers, ripper blades, rock saw or backhoe (Koch et al. 2010). The depth of the trench will vary according to soil conditions; however, it is believed that a trench of 1.2 to 2 m depth is usually sufficient to disrupt root grafts (Cook 2012; Haugen et al. 2007; Juzwik et al. 2011). When performing the root disruption, special attention should be given in order to not injure surrounding healthy trees as it could further spread the disease.
If all oak trees inside the trench line are dead or dying, the trench can be refilled right away as new root grafts will not re-form (Wysong and Sharon 1986). However, there is a potential risk for roots to re-graft when many healthy oak trees are inside the trench line. The use of trench inserts can be considered to further reduce the risk of re-grafting (Wilson and Lester 2002; Wilson 2005). Water permeable inserts (for example, Typar Biobarrier) are recommended over water impermeable membrane. Because of their cost, the installation of an insert are rarely used as a management option (Juzwik et al. 2011).
Alternatively to root disruption, root rupture can be performed when the infested area is small or no equipment is available or able to perform the trenching because of topography and soil conditions. Root rupture involves using an excavator to rip out and overturn the stumps and associated roots masse to break the root graft. This procedure has been reported to have more than 90% effectiveness in stopping the spread of the pathogen (Haugen et al. 2007).
10.3.2 Tree removal
Limiting the production of spore mats on red oak wilt infected trees must be part of the control strategy to effectively manage oak wilt. It is important to note that the removal of living trees infected with oak wilt should only occur after interconnected roots have been disrupted to prevent further spread into adjacent trees. Infected red oak trees that died in the summer should be removed before the following spring (prior to May) to prevent new spore mat formation (Wilson 2005). After trees are cut down, they should be subject to sanitation measures as soon as possible. Sanitation measures are recommended for all material 7.6 cm in diameter or larger (Bruhn 1995). Many disposal and treatment options are deemed suitable to hinder spore mat production including:
- burning or burying
- debarking or splitting, and drying the wood
- steaming or kiln-drying wood or lumber
- process logs into lumber or chips (for mulching or for use as fuel) or
- process logs into firewood (firewood shall be covered with plastic and sealed tightly to the ground between April and August to ensure vectors can't access the fungal mats and to kill the fungus (Cook 2012; Jones 1973; O'Brien et al. 2011; Roberts 2016b).
Any other process that would dry the wood before spore mat formation would be acceptable (Juzwik et al. 2011). The fungus is not able to survive in wood with a moisture content below 20% (Wysong and Sharon 1986). Girdling and debarking of the lower part of the trunk is not a reliable treatment method to control spore mat formation (Juzwik et al. 2011).
Asymptomatic trees within the infected area need to be cut and felled; however, they do not require any treatments. Oak tree that died more than a year ago do not need to be subject to special sanitation measures as they no longer produce spore mats (Bruhn 1995). Also, as spore mats are very rarely produced on trees of the white oak group, special treatment or disposal methods are not required (Cook 2012).
If infected or dead trees are felled outside the period when key vectors are most likely to spread the pathogen (i.e. August to March), the movement of the wood from its original location to another location for disposal or treatment may not require any special phytosanitary measures as the risk of spreading oak wilt would be marginal. This would be true as long as the sanitation measures are completed prior to next spring.
10.3.3 Chemical treatment
Since the 1950's many different compounds have been tested in the U.S. to control oak wilt with limited success. Triazole-type products (for example, propiconazole) have been the most studied and appear to be the most commonly used product in the U.S. (Wilson 2005).
As a fungistatic compound, propiconazole inhibits B. fagacearum growth without killing it (Wilson and Forse 1997). Systemic injection of propiconazole into the tree vascular system has been demonstrated to be a treatment option to reduce the impact of oak wilt on oak trees. Treatment with this propiconazole is used mostly for high value trees around the expanding infection center as it is considered to be a costly control option (Wilson 2005). The effectiveness of the treatment is dependent on many factors including the disease progression, the oak species, tree size and the application method and timing (Koch et al. 2010). This fungicide treatment is mostly used as a preventative treatment but also as a therapeutic treatment in some cases. Propiconazole does not prevent infection of red and live oaks but there is some evidence that it may prevent some white oak infection (Koch et al. 2010). For all oaks, it has been reported to be an effective treatment to slow down symptom development and to reduce tree mortality, especially for white oaks (Koch et al. 2010). In general, treatment with propiconazole has been demonstrated to prevent the production of fungal mats on oak tree killed by oak wilt (Koch et al. 2010).
Oak wilt fungus can survive at least 3-4 years in the root system (Blaedow et al. 2010; Gibbs and French 1980). Since the concentration of propiconazole decline overtime, retreatment is recommended every 2 years to maintain disease suppression (Blaedow et al. 2010). It remains unknown if retreatment could eventually lead to disease eradication. Therefore, treated trees on the edge of infection centers should be considered to have latent infection and should be regarded as potentially infected.
Studies have demonstrated that propiconazole moves well upward but only marginally downward in the roots systems when applied via intravascular injection. The presence of the fungicide in the upper part of the tree may greatly reduce the progression of the disease to above-ground parts and the development of symptoms without preventing infection through below ground root grafts (Blaedow et al. 2010). For these reasons, the treatment is not curative and does not stop dispersal of the fungus through the root system (Blaedow et al. 2010). Therefore, propiconazole is not a good replacement to root disruption and should be considered as a tool to suppress the disease rather than to prevent or eradicate the disease (Juzwik et al. 2011; Wilson and Lester 2002; Wilson 2005).
The product "Alamo" (propiconazole) is used for oak wilt control in the U.S. According to preliminary discussion with Syngenta, this product will not likely be supported by Syngenta for use in Canada. No fungicides for oak wilt are currently registered in Canada.
10.4 Management strategy
10.4.1 Analysis of the incursion
Following the detection of oak wilt disease in Canada, an analysis of the incursion will be conducted.
Key factors to be taken into account include:
- Number of infested trees based on the results of the delimitation survey
- Infested site characteristics (location and size)
- Environment around the infested site (oak prevalence, forest cover vs urban area)
- Number of risk generators (stakeholders that handle regulated articles) including; woodlot owners, forest industries, arborists, firewood vendors, members of the public etc.)
- Magnitude and complexity of movement of regulated articles out of the infested area
Based on the analysis of the incursion, an active or passive control strategy may be implemented in response to the oak wilt detection.
10.4.2 Active control
Active control should be the preferred approach when the analysis of the incursion demonstrates that containment and control of the natural spread of oak wilt is feasible based on a limited number of infested trees in a defined area.
Two methods may be applied for active control of oak wilt:
- Root disruption (soil trenching or root rupture)
- Tree removal and disposal
10.4.3 Passive control
Passive control should be the preferred approach when the analysis of the incursion demonstrates that the infestation is too extensive to successfully control the natural spread of oak wilt.
Passive control is implemented to slow the human-assisted spread of the disease and protect non-infested areas by controlling the movement of regulated articles.
Under a passive control strategy, a regulated area will be established around the place(s) where oak wilt is known to occur. Movement of regulated articles (firewood, logs, branches and bark) outside of this area will be prohibited.
The regulated area will be dependent on the extent of the infestation in Canada.
Regulatory boundaries must be well communicated to the risk generators and general public to promote compliance to established regulations.
10.4.4 Evaluation of management options
Taking into account information from 10.4.1 and the advice of the Oak Wilt Technical Advisory Committee, financial and human resources available from all relevant parties will be considered, assessing operational capacity and opportunities for collaboration to support and execute the preferred management strategy. Using these inputs, the optimal control methods selected for implementation can then be recommended to governing senior officials.
The approach should be reviewed on an annual basis and adjustments will be made based on the best available information.
Canadian research will strive to address knowledge gaps relevant to protecting the oak resource in Canada. Potential research projects may be designed to advance survey and monitoring methodologies, diagnostic techniques, control tactics or management strategies.
In an effort to promote a collaborative and coordinated approach, information on current or future research proposals can be shared with the Chair of the Oak Wilt Technical Advisory Committee for inclusion in the table below. The table will be updated on a regular basis to reflect historical and ongoing efforts.
|Project title||Project lead(s)||Expected outcomes|
|Detection of Oak wilt using metagenomics||Guillaume Bilodeau||
|Flight behavior, degree day models, and identification of potential nitidulid beetle vectors of oak wilt along the northern range of oaks in Canada||Dr. Sharon Reed (OMNRF), Dr. Jon Sweeney (CFS)||
Specific to northern range:
|Flight behavior, degree day models, and identification of nitidulid beetle vectors of oak wilt in southern Ontario||Dr. Sharon Reed (OMNRF), Jennifer Llewellyn (OMAFRA)||Specific to southern Ontario: Identification of nitidulid species attracted to freshly wounded red and burr oaks, data on the start, end, and peak flight periods of each nitidulid species, confirmation of degree day models already built, development of new degree day models for other pertinent species.|
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