RMD-10-27: Pseudomonas syringae pv. aesculi (horse chestnut bleeding disease)

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Issued: 2010-11-23

Preface

As described by the International Plant Protection Convention (IPPC), Pest Risk Analysis (PRA) includes three stages: initiation, pest risk assessment and pest risk management. Initiating the PRA process involves identifying pests and pathways of concern and defining the PRA area. Pest risk assessment provides the scientific basis for the overall management of risk. Pest risk management is the process of identifying and evaluating potential mitigation measures which may be applied to reduce the identified pest risk to acceptable levels and selecting appropriate measures.

This Risk Management Document (RMD) includes a summary of the findings of a pest risk assessment and records the pest risk management process for the identified issue. It is consistent with the principles, terminology and guidelines provided in the IPPC standards for pest risk analysis.

Table of Contents

1.0 Summary

Horse chestnut bleeding disease is a newly identified disease that kills the horse chestnut tree Aesculus hippocastanum L.. The disease has been found in parts of Europe including France, Belgium, Germany, Italy, the Netherlands, and the United Kingdom. Originally, the presence of bleeding cankers in horse chestnut trees was believed to be associated with fungal pathogens (Phytophthora species). It is only recently that the disease has been associated with Pseudomonas syringae pv. aesculi., a bacterium first described in the 1970's as causing leaf spot on the Indian horse chestnut tree, Aesculus indica.

P. syringae pv. aesculi is not known to occur in North America. Only one native population of Aesculus is known to occur in Canada (A. glabra on Walpole Island in Lake St. Claire, ON). However, A. hippocastanum and A. glabra (Ohio buckeye) have been planted in North America including in Canada as an amenity tree, cultivated mostly as a shade tree in public areas. In addition to the direct threat to North American specimens of horse chestnut, incursion into Canada of P. syringae pv. aesculi could also negatively influence horticultural trade, especially with the United States.

Although research is underway, little is currently known about the disease. In addition, uncertainty exists with respect to the potential for this pathovar to spread to other native and non-native species. It is known that where the pathovar does occur in Europe, infection rates and losses of horse chestnut trees are high.

This RMD provides notification that P. syringae pv. aesculi is a quarantine pest to Canada and that the Canadian Food Inspection Agency (CFIA) will regulate the importation of Aesculus spp. plants for planting including seed to prevent the introduction of this pathovar to Canada.

2.0 Purpose

The purpose of this document is:

  • to seek stakeholder input
  • to inform stakeholders
  • to record a risk management decision
  • other (explain)

3.0 Scope

This RMD describes the risks associated with the introduction of the causal agent of horse chestnut bleeding disease, Pseudomonas syringae pv. aesculi, into Canada and outlines the risk management decisions taken by the Canadian Food Inspection Agency (CFIA) regarding this pest.

Information pertaining to current import requirements for specific plants or plant products may be obtained from the Canadian Food Inspection Agency (CFIA) Automated Import Reference System (AIRS).

4.0 Definitions

Definitions for terms used in this document can be found in the Plant Health Glossary of Terms or the IPPC Glossary of Phytosanitary Terms.

5.0 Background

Recent scientific literature identifies P. syringae pv. aesculi as a destructive plant pest that seriously affects and leads to the death of plants in its host genus Aesculus spp.. This pathovar of P. syringae was originally observed in Aesculus indica in India, where it caused only a minor leaf spot. It is now rapidly spreading through Europe, affecting trees in France, Belgium, Germany, Italy, the Netherlands, and the United Kingdom. Recent surveys have estimated that in the United Kingdom approximately 49% of horse chestnut trees are infected, as are 39% in the Netherlands. Once established in the host, the disease cannot be successfully managed. In June 2009, this pest was added to the European Plant Protection Organization alert list (EPPO 2009).

On January 7, 2010 the United States Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) issued a Federal Order prohibiting the importation of Aesculus spp. plants for planting, with the exception of seed, from all countries except Canada. Aesculus spp. have been added to the United States' list of plants for planting whose importation is not authorized pending pest risk analysis (NAPPRA).

The decision to exempt Canada from these restrictions was made after APHIS was informed that the CFIA is also considering taking measures to mitigate the risk of introducing P. syringae pv. aesculi from off-continent. P. syringae pv. aesculi is not known to occur in Canada.

Under previous versions of the CFIA's directive D-01-01: Phytosanitary Requirements to Prevent the Entry and Spread of Phytophtora ramorum, Aesculus spp. were prohibited entry into Canada from countries where P. ramorum is known to occur, with an exception for the Netherlands. In July 2008, the directive was amended to regulate most of the hosts by species rather than by genera. Today, the only Aesculus spp. that are prohibited entry from most of Europe are A. californica and A. hippocastanum. All species are permitted entry from the Netherlands under a CFIA Approved Pest Free Certification Program for Phytophthora ramorum.

While hosts of P. syringae pv. aesculi have not been extensively planted in Canada, the introduction of this pathovar could have negative repercussions on the horticultural trade, principally with the United States. In addition, uncertainty exists with respect to the potential for this pathovar to spread to other native and non-native species in Canada and the United States. Once established, damage is significant and affected trees must be cut rapidly as the wood quickly weakens and becomes a hazard.

6.0 Pest Risk Assessment Summary

A pest risk assessment (Miller, 2010) was completed by the CFIA in April 2010 to evaluate the potential risks related to the introduction into Canada of P. syringae pv. aesculi, the pathovar causing horse chestnut bleeding disease. The following sections present the highlights of this assessment.

6.1 Pest Biology

6.1.1 Life History

At first, the cause of horse chestnut bleeding disease was believed to be a Phytophthora species, but investigations have showed that the bacterium P. syringae pv. aesculi is consistently associated with the disease. P. syringae is a broad-based grouping of bacteria containing many well known pathovars (a bacterial strain or set of strains with the same or similar characteristics).

Data is lacking on the geographical distribution, biology, epidemiology, and host range of P. syringae pv. aesculi; inferences must therefore be drawn from other closely related taxa. For example, the related pathovar P. syringae pv. syringae van Hall has a broad host range and is widely distributed around the world, demonstrating adaptability and tolerance for different climatic conditions. It affects all parts of the plant from the seedling stage to maturity and survives on a number of crop and non-crop species, which serve as sources of primary inoculum for infection. It is transmitted and disseminated through living plants and vegetative propagative material as well as by wind-driven rain. It can be assumed with reasonable confidence that P. syringae pv. aesculi will behave in a manner similar to other P. syringae pathovars.

6.1.2 Host Range

A. hippocastanum (both white and red cultivars) is most affected by P. syringae pv. aesculi and A. hippocastanum cv. Baumanii appears to be particularly susceptible. A. glabra may prove to be susceptible and research is currently being carried out on the susceptibility of other species (e.g. A. x mutabilis, A. flava, A. parviflora and A. pavia).

6.1.3 Symptoms

Early symptoms tend to be limited to bleeding lesions; scattered drops of rusty-red, yellow-brown or almost black, gummy liquid ooze from small or large patches of dying bark on the stems or branches of infected horse chestnuts. Trees which have been infected for a period of time may show a lesion on the bark, which oozes a resin on to the trunk or sometimes the branches. Bleeding patches may be associated with the base of the tree at the soil surface or may start higher up the trunk at about one metre, and then extend upwards. Under dry conditions during the summer, this exudate dries to a dark, brittle crust.

After some months, the centre of the bleeding bark patch may become cracked. In time, fruit bodies of wood-rotting fungi often appear on the surface of the dead bark, protruding out of the bark cracks.

6.2 Distribution

This plant pest was first observed in Aesculus indica, causing leaf spots in India but no further information could be found on the extent or severity of the disease it may cause in India or in other Asian countries (EPPO 2009). How this pest arrived in Europe is uncertain, but the most likely route is through trees intended for planting in Britain. P. syringae pv. aesculi is reported from, but not necessarily limited to, the United Kingdom (England, Scotland and Wales), the Netherlands, Belgium, Germany, France and Italy.

6.3 Pathways

The means of transmission of P. syringae pv. aesculi remain uncertain. The pathovar has been isolated from the surface of horse chestnut leaves and branches, as well as from flowers and various parts of the fruit. Bacteria were also detected in rainwater in the vicinity of diseased trees. However, more studies are needed to determine the possible role of water, insects or even human activities (e.g. pruning) in disease transmission.

Natural, local spread of other P. syringae pathovars is generally achieved via rain splash and wind-driven rain. Spread over long distances is more easily accomplished via human activity involving the movement of P. syringae-infested propagative material. This would most likely include, but not necessarily be limited to, trade in horticulture stock (plants for planting, seed, etc.). The risk mitigation measures proposed in this RMD address long-distance spread via the movement of infected horticultural stock.

The pathovar is likely to be cryptic on dormant A. hippocastanum stock and on A. indica, in which the pathovar only causes a mild leaf spot (where leaves would not be available for visual inspection). P. syringae pv. aesculi is difficult to detect if there is absence of canker. With respect to A. hippocastanum, any size of tree can be affected.

Planting material of Aesculus spp., particularly A. hippocastanum and possibly A. indica, has a high likelihood of being infested with P. syringae pv. aesculi when sourced from Europe or India.

6.3.1 Establishment Potential

Several of the more southerly North American Aesculus species (as well as the Eurasian A. hippocastanum) grow north to zone 5, which includes most of coastal British Columbia, southern Ontario, and southern Nova Scotia. As a group, many tend to grow in cultivation farther north than their natural occurrence. A. indica grows naturally in the western Himalayas. It is uncertain if it is grown in Canada but it is reported to be hardy to zone 7, suggesting that it would probably survive in a small portion of southwestern British Columbia.

As their climate is similar to that of the affected parts of Europe, zones conducive to the survival of A. hippocastanum in Canada would, in all likelihood, be suitable for establishment of the pathovar as well. The pathovar has already demonstrated an ability to adapt to a new host by spreading from A. indica, in which it was originally found, to A. hippocastanum. High uncertainty exists as to which other hosts, such as Ohio buckeye (the only native Canadian Aesculus species), may prove to be susceptible.

6.3.2 Spread Potential

To date, the epidemiology of the disease remains poorly understood. Inference can be made from the study of other closely related taxa. The spread of related plant pathogenic bacteria largely depends upon the movement of contaminated water droplets via wind and water splash. There is also evidence that tree-to-tree infections can occur via root systems, grafted or otherwise.

Recently, U.K scientists sequenced the genome of Pseudomonas syringae pv. aesculus. The results made them suspect that the spread of this very aggressive disease may have resulted from a single introduction.

Generally, diseases caused by members of the P. syringae complex tend to be favored by wet, cool conditions (optimum temperatures for disease tend to be around 12-25°C). They also tend to be seed-borne and are likely easily dispersed naturally.

6.4 Potential Economic and Environmental Consequences

Assigning the potential economic impact of P. syringae pv. aesculi is difficult. A. hippocastanum, the most affected species, is not common in Canada. It has been imported and planted as an amenity tree, as it is valued as an ideal shade tree, but has rarely become naturalized. A cost would be related to removing and replacing affected trees. The loss of this species would therefore be felt in some locations, but the overall impact would be relatively low.

The introduction of this pest in Canada would however likely have a negative impact on the horticulture trade, especially with the United States. USDA has recently exempted Canada from Federal Order DA-2010-02 regulating P. syringae pv. aesculi, which prohibits the entry of Aesculus spp. from all other parts of the world. This decision was made after APHIS was informed that the CFIA is also in the process of taking measures to mitigate the risk of introducing P. syringae pv. aesculi. There is an opportunity for the USDA and the CFIA to harmonize their phytosanitary requirements in order to prevent the entry of this pathovar to North America and to allow Aesculus spp. plant material to continue moving between Canada and United States.

7.0 Risk Management Considerations

P. syringae pv. aesculi is a serious pathovar with a high spread potential. Much remains unknown with regard to the disease including the potential susceptibility of various native and non-native tree and plant species that might be potential hosts of P. syringae pv. aesculi (including North American members of the buckeye family).

In addition, while the major host, A. hippocastanum, has not been extensively planted in Canada and naturalization is considered very rare, the introduction of P. syringae pv. aesculi could negatively influence horticultural trade, especially with the United States. The USDA has added Aesculus spp. to their NAPPRA list and instigated restrictions against the importation of Aesculus spp. from countries other than Canada, with the understanding that the CFIA is concurrently reviewing the Canadian regulation of this pest.

P. syringae pv. aesculi has only emerged relatively recently as a concern and much about it is still poorly understood. Given this the lack of information and the adaptability and the potential virulence of this pathovar, the CFIA wishes to prevent the introduction of P. syringae pv. aesculi into Canada.

8.0 Risk Management Options

8.1 Option 1: Do not regulate Pseudomonas syringae pv. aesculi and do not prohibit the importation of Aesculus spp. from any countries

Advantages:
  • No additional costs for the CFIA.
Disadvantages:
  • No authority to require phytosanitary measures preventing the introduction of P. syringae pv. aesculi into Canada.
  • Threat to the North American A. hippocastanum population.
  • Potential threat to other native and non-native species, as there are still many unknowns related to this disease.
  • Incursion into Canada of P. syringae pv. aesculi would negatively influence exports of horticultural material, especially to the United States.

8.2 Option 2: Regulate P. syringae pv. aesculi and prohibit the importation of Aesculus spp. only from countries where the pest is known to occur

Advantages:
  • By regulating P. syringae pv. aesculi, the CFIA would have the authority to require phytosanitary measures preventing the introduction of this pathovar into Canada.
Disadvantages:
  • Same disadvantages as Option 1, as the exact distribution of P. syringae pv. worldwide is uncertain.

8.3 Option 3: Regulate P. syringae pv. aesculi and prohibit importation of Aesculus spp. from all countries except the United States, pending the identification, evaluation and acceptance of appropriate risk mitigation measures (e.g. area freedom based on official surveys)

Advantages:
  • By regulating P. syringae pv. aesculi the CFIA would have the authority to require phytosanitary measures preventing the introduction of this pathovar into Canada.
  • Reduces the threat to the North American A. hippocastanum population.
  • Limits the potential threat to other native and non-native species, as they could potentially be affected.
  • Avoids any negative impact on exports of horticultural material related to the incursion of P. syringae pv. aesculi into Canada.
Disadvantages:
  • Minor additional resources on the part of the CFIA would be needed to put these regulations in effect.

9.0 Pest Risk Management Decision

The CFIA intends to adopt Option 3. This RMD provides notification that the CFIA will prohibit the importation of Aesculus spp. plants for planting, including seed, from all countries except the United States. The Directive D-08-04 Plant Protection Import Requirements for Plants and Plant parts for Planting: Preventing the entry and spread of regulated plant pests associated with the plants for planting pathway will be updated to reflect these new requirements. This prohibition is necessary to prevent the introduction and dissemination in Canada of P. syringae pv. aesculi, the causal agent of horse chestnut bleeding disease, a harmful plant pathovar. These new regulations will be effective 30 days after distribution of this RMD to stakeholders.

10.0 Stakeholder Communications

Follow-up communications will be required to notify the following trading partners and stakeholders of changes in import regulations resulting from the risk management decision:

  • Trading partners (including via a World Trade Organization notification, the International Plant Protection Convention Phytosanitary Portal, etc.);
  • CFIA Program Officers, inspection staff, Import Service Centers, etc.;
  • Other government organizations (e.g. the Canadian Border Services Agency); and
  • Industry stakeholders.

11.0 References

Canadian Food Inspection Agency. Plant Protection Import Requirements for Plants and Plant Parts for Planting: Preventing the Entry and Spread of Regulated Plant Pests Associated with the Plants for Planting Pathway (D-08-04).

Canadian Food Inspection Agency. Phytosanitary Requirements to Prevent the Entry and Spread of Phytophthora ramorum (D-01-01).

EPPO. 2009. Internet Bulletin: Pseudomonas syringae pv. aesculi.

Kinver, M. 21 April 2010, BBC Science & Environment: DNA boost in battle against bleeding canker.

Miller, S. J. 2010. PHPD Request No. 06-09. Plant Health Risk Assessment: Horse Chestnut Bleeding Disease, as caused by Pseudomonas syringae pv. aesculi, Plant Health Risk Assessment Unit, Canadian Food Inspection Agency.

USDA. 2010. USDA Plants Database. Plants profile: Aesculus hippocastanum L., horse chestnut

12.0 Endorsement

Approved by:

Chief Plant Health Officer

Date modified: