RMD-13-04: Consolidated Pest Risk Management Document for pest plants regulated by Canada
Appendix 6B: Risk Management Considerations for Dioscorea polystachya (Chinese yam)

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History of invasiveness

In the 25 or so years that Dioscorea polystachya has been present in the U.S., it has spread across many of the eastern states. In that short period of time, it has been identified as a species of concern by various state invasive plant councils:

As described in Crooks and Soulé, 1999, some invasive plant populations have historically shown a "lag" period after first establishing, during which the population remains relatively small and has a low impact. Following the lag, the population then explodes, its range and impacts rapidly increasing. In brief, "past performance of an exotic is a poor predictor of potential population growth, range expansion and ecological impact" (Crooks and Soulé, 1999).

The CFIA is concerned that this may be the case with Dioscorea polystachya and that the early reports of invasiveness described above may be signs that the plant could become a much more significant threat in the future, both in the U.S. and in Canada.

Potential Mitigation Measures for Natural Means of Dispersal

Though populations are present in U.S. states that adjoin the U.S.-Canada border (see Figure 1), Dioscorea polystachya has only been recorded, within those states, in counties that are not immediately adjacent to the U.S.-Canada border (USDA-NRCS, 2009). Natural dispersal is therefore not expected to be an important factor in the spread of Dioscorea polystachya to Canada. If populations continue to spread in the U.S., however, dispersal of floating bulbils via waterways may become a concern and therefore the species can easily spread into nearby riparian areas. This means of spread would be extremely difficult to control.

Potential Mitigation Measures for Intentional Introduction Pathways

Plants for Planting Excluding Seed

Previous imports

Based on the information available in the CFIA's Import Permit System, Import Retrieval System, and information compiled at the CFIA's Import Service Centres, Dioscorea polystachya may have been recently imported into Canada. Three Permits to Import were issued by the CFIA over the last five years for cuttings of plants from the genus Dioscorea, but the precise species were not indicated. No plants from the genus Dioscorea are listed as available in Canadian nurseries (CNLA, 2009).

Potential risk mitigation measures

Non-regulatory measures

Regulatory measures

Allow sale of Dioscorea polystachya with special conditions such as not to be grown close to natural areas and mandatory control of adventives. Allowing sale with special conditions is not considered effective because once grown in private gardens, CFIA does not have adequate resources to monitor.
Regulate Dioscorea polystachya under the Plant Protection Act as a quarantine pest:

Trade Implications

As described above under Previous imports, Dioscorea polystchya has only rarely been imported into Canada. Prohibiting the species should therefore not significantly impact Canadian importers and vendors. In addition, other ornamental species could potentially be substituted. If the intent is cultivation for consumption or for medicinal purposes, roots (processed or dried) could be imported rather than grown in Canada (see Section below. Plant parts for consumption or for medicinal use, for more information).

Cost-effectiveness and Feasibility

It may be useful for CFIA inspectors to check imported Dioscorea plants to ensure that Dioscorea polystachya is not being imported under an incorrect name or synonym. This is particularly the case with Dioscorea polystachya because it has been widely sold in the U.S. as Dioscorea polystachya. This would require training inspectors and providing them with identification material. As described above under Previous imports, however, Dioscorea plants are only rarely imported; the impact on CFIA resources should therefore be minimal.

Seed

Previous imports

There is no record in the information available in the CFIA's Import Permit System, Import Retrieval System, and data compiled at the CFIA's Import Service Centres that Dioscorea polystachya seed has ever been imported to Canada. Seed of other Dioscorea species is known to have been imported at least three times in recent years.

Potential risk mitigation measures

Regulate Dioscorea polystachya as a prohibited noxious weed (Class 1) under the Weed Seeds Order of the Seeds Act Footnote 2.

Regulate as a quarantine pest under the Plant Protection Act. Add this species to the List of Pests Regulated by Canada (CFIA 2009):

Sufficient information about the use of Dioscorea polystachya as a potential crop does not exist. If the proponent, located in Canada, needs to collect more information about the plant (e.g. to generate data for a determination of environmental safety), then confined research field trials under Part V of the Seeds Regulations could be authorized by the Plant Biosafety Office.

Trade Implications

As described above under Previous imports, Dioscorea polystachya seed has only rarely been imported to Canada. Prohibiting the species should therefore not significantly impact Canadian importers and vendors. In addition, other ornamental species could potentially be substituted. If the intent is cultivation for consumption or for medicinal purposes, roots (processed or dried) could be imported rather than grown in Canada (see Section below. Plant parts for consumption or for medicinal use).

Cost-effectiveness and Feasibility

The CFIA Seed Program is already in place to prevent the entry of prohibited noxious weeds. As well, as described in Section Appendix 6A, Probability of Entry, Dioscorea polystachya is not known to be a weed of crops. Negligible effort would therefore be required to identify the seeds or take action on non-compliant shipments because it would be unlikely to find Dioscorea polystachya seeds as a contaminant in sampled seed or grain lots.

It may be useful for the CFIA to occasionally sample imported Dioscorea seeds to ensure that Dioscorea polystachya seeds are not imported under an incorrect name or synonym. As described above under Previous imports, however, Dioscorea seeds are only rarely imported; the impact on CFIA resources should therefore be minimal.

Plant parts for consumption or for medicinal use

Background

Importers are required to specify the intended end-use (e.g., consumption, planting) of imported products when requesting a Permit to Import, and a Permit to Import issued for plant parts imported for consumption specifies that the plant parts cannot be used for propagation. However, once the plant parts are distributed, the CFIA has no control over how they are used. Dioscorea polystachya tubers imported for consumption could therefore conceivably be planted for cultivation as an ornamental, or to grow tubers for consumption or medicinal use.

Previous imports

Based on the information available in the CFIA's Import Permit System, Import Retrieval System, and information compiled at the CFIA's Import Service Centres, Dioscorea polystachya tubers for consumption or medicinal use may have been recently imported into Canada. Only one Permit to Import was issued by the CFIA over the last five years for tubers for consumption from the genus Dioscorea, but the precise species was not specified. No shipments of Dioscorea polystachya tubers are known to have been imported, though approximately a dozen shipments of tubers of other Dioscorea species have been imported in recent years.

Potential risk mitigation measures

Non-regulatory measures

Encourage voluntary cessation of the sale of Dioscorea polystachya. This would not be effective by itself, but could support other measures.

Regulatory measures

Regulate Dioscorea polystachya under the Plant Protection Act as a quarantine pest and, as a supporting measure, prohibit importation of tubers for consumption.

Trade Implications

As described above under Previous imports, Dioscorea polystachya plant parts for consumption or medicinal use have only rarely been imported to Canada. Prohibiting the species should therefore not significantly impact Canadian importers and vendors.

Cost-effectiveness and Feasibility

It may be useful for CFIA inspectors to occasionally check imported Dioscorea tubers to ensure that Dioscorea polystachya is not being imported under an incorrect name or synonym. This would require training inspectors and providing them with identification material. As described above under Previous imports, however, Dioscorea tubers are only rarely imported; the impact on CFIA resources should therefore be minimal.

Potential Mitigation Measures for Non-intentional Introduction Pathways

Note that, as described in Section Appendix 6A, in the section called Probability of Entry, unintentional introduction pathways (through contaminated soil, seed, grain, etc.) are considered only minor pathways, posing a minimal risk to Canada. These pathways will therefore not be detailed here.

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