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

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Identity of Organism

Name: Dioscorea polystachya Turcz. (Family Dioscoreaceae) (USDA-ARS, 2009)

Synonyms: Dioscorea batatas Decne., Dioscorea cayenensis Lam. var. pseudobatatas Hauman, Dioscorea decaisneana Carrière, Dioscorea doryphora Hance, Dioscorea opposita auct., Dioscorea oppositifolia auct., Dioscorea potaninii Prain & Burkill, Dioscorea rosthornii Diels, Dioscorea swinhoei Rolfe, Dioscorea trinervia Roxb. ex Prain & Burkill (Global Invasive Species Database, 2009; USDA-ARS, 2009)

English common names: Chinese yam, Chinese-potato, cinnamon-vine (USDA-ARS, 2009)

French common name: Igname de Chine (CAB International, 2007; USDA-ARS, 2009)

Description: The plants are vines growing from spindle-shaped tubers which are on long stalks and are deeply buried. The twining stems are up to 5 m in length. Small bulbils (less than 2 cm in diameter) are produced in the leaf axils. The lower leaves are alternate, becoming opposite higher up the stem and are 3–9 cm long × 3–11 cm wide. The petioles are as long as the blade. The blade is 7(–9)-veined, glabrous and 3-lobed. Inflorescences are borne in the leaf axils. The flowers are small, yellowish and have a cinnamon fragrance. Staminate and pistillate flowers are on separate plants. The fruit is an ovate capsule. The seeds are winged. Only one pistillate specimen has been documented from North America and it is assumed that the plants propagate vegetatively by means of the bulbils (FNA Editorial Committee, 1993+; Gleason and Cronquist, 1963).

Dioscorea polystachya is native to eastern Asia, where it is cultivated for its edible tubers. In North America, it is planted as a garden ornamental and has become naturalized throughout much of the eastern United States (FNA Editorial Committee, 1993+).

Organism Status

Dioscorea polystachya has been imported for sale in Canada on a very limited scale. The UBC Botanical Garden offered it for sale (as Dioscorea batatas) in their catalogue in 2006. There are no records of plants outside of cultivation. Based on this information, for the PRA area, this species is considered to be possibly present only in cultivation.

Current Regulatory Status

Dioscorea polystachya is not regulated in Canada. It is not regulated as a federal noxious weed in the U.S., nor is it regulated in any of the states (USDA-NRCS, 2009).

It is currently listed in the Southeast Exotic Pest Plant Council's Invasive Exotic Pest Plant List for Tennessee as a Rank 1-Severe Threat species, indicating that it is an exotic species that possesses characteristics of an invasive species and could spread easily into native plant communities and displace native vegetation (Tu, 2002).

Probability of Entry

Dioscorea polystachya is native to eastern Asia, where it is cultivated for its edible tubers. In North America, it is planted as a garden ornamental and has become naturalized throughout much of the eastern United States (FNA Editorial Committee, 1993+).

Table 1: Summary of pathways for Dioscorea polystachya (Chinese yam)
Type of pathways Specific pathways
Natural dispersalNaturalspread is by seeds and aerial bulbils, but sexual reproduction by means ofseeds has not been detected in North America. The aerial bulbils can bespread by rodents who feed on them. Thispathway allows only local movement and is not likely to be a conduit forentry into Canada.
Intentional introduction Plantshave been intentionally introduced into the U.S. as an ornamental ormedicinal plant. In eastern Asia it has been widely planted as a starchy foodtuber crop. This isthe most likely pathway for entry into Canada.
Unintentional introduction As withany weed, introduction is possible via soil imported with nursery stock frominfested areas of the U.S. The riskfrom this pathway is likely to be minor, however. Asreproduction does not appear to occur through seed in North America,contaminated seed imported from the U.S. is not a potential pathway. Seed imported from other countries wheresexual reproduction does occur could be contaminated, however. The risk from this pathway is likely to beminor, though, as the plant is not known to be a weed of crops and so wouldprobably not be harvested along with seeds for planting.

Probability of Establishment

Dioscorea polystachya is native to China (Anhui, Fujian, eastern Gansu, northern Guangdong, Guangxi, Guizhou, Hebei, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Jilin, Liaoning, southern Shaanxi, Shandong, Sichuan, northern Yunnan and Zhejiang), Japan (Hokkaido, Honshu, Kyushu, Shikoku), Korea and Taiwan (USDA-ARS, 2009). The species is widely cultivated for food in temperate eastern Asia (Bailey and Bailey, 1976).

The species is introduced and cultivated as an ornamental, food and medicinal plant and is naturalized in temperate regions, including the U.S. (USDA-ARS, 2009). It is described as root hardy to USDA Plant Hardiness Zone 5 (Bailey and Bailey, 1976).

In its native range, Dioscorea polystachya grows in forests, scrub, herbaceous plant communities, on mountain slopes, along rivers and roadsides (Wu and Raven, 2000).

Dioscorea polystachya has not become established outside cultivation in Canada (CFIA, 2008; Scoggan, 1979).

Dioscorea polystachya has a wide range of environmental adaptability and few pests and predators in North America. It has a high degree of asexual reproductive vigour, and is difficult to manage once firmly established (Tu, 2002).

Figure 1: Range of Dioscorea polystachya (Chinese yam) in North America

Figure 1. Description follows.
Description for Figure 1:

This map shows the range of Dioscorea polystachya in North America indicating its presence through the use of the colour green. The eastern half of the United States is almost completely covered in green along with a fifth of the lower western half.

Source : USDA-NRCS, 2009.

Figure 2: Potential range of Dioscorea polystachya (Chinese yam) in Canada

Potential range of Dioscorea polystachya (Chinese yam) in Canada
Description for Figure 2:

This image shows the potential range of Dioscorea polystachya in Canada and northern America through the use of a map. Red is used to indicate the regions in which Centaurea iberica could survive according to the Canadian Plant Hardiness Zones map, in this case NAPPFAST Hardiness Zones 5-9. In Canada, Dioscorea polystachya is likely to become weedy or invasive in parts of Canada, including southern and coastal British Columbia, southern Ontario, southern Quebec and parts of the Maritime Provinces. Most of the red can be found in the U.S., this includes the greater parts of the eastern and western coasts as well as areas located near the Great Lakes extending downwards beyond the image.

Source : USDA-NRCS, 2009.

Probability of Spread

Dioscorea polystachya is found in thickets, ravines, stream banks, creek bottoms, limesinks, granite outcrops, alluvial woods, roadsides, drainage canals, waste places and fence rows in its introduced range in the southeastern U.S. (FNA Editorial Committee, 1993+).

Initial infestations are generally associated with human disturbances, such as old home sites and along roadways. From these areas, the species can easily spread into nearby riparian areas and undisturbed habitats. In 1970, the species had not yet been documented as escaping from cultivation in the U.S., but by 1986 it had become naturalized (Tu, 2002).

As Dioscorea polystachya can grow in both disturbed and natural sites, habitats will not be a limiting factor for spread in Canada.

Potential Economic Consequences

Both the tuber and bulbils are edible, although the bulbils are not generally used as food. The edible tuber, which can measure up to 1 m long and weigh up to 2 kg or more, is flavourful and nutritious. The tubers are sometimes used in herbal medicine. The species is frequently planted for its ornamental value (Tu, 2002).

In forested areas, branches can be broken off of trees by the weight of the vines (ISSG, 2009).

Manual and mechanical methods of plant removal can effectively control small isolated patches of Dioscorea polystachya. However, these methods are extremely time and labour-intensive, as the large, deeply-buried tuber makes removal very difficult. Herbicide application appears to be the most effective means for controlling large infestations. Repeated treatments are usually necessary to completely kill large underground tubers (ISSG, 2009), which increases the cost.

Potential Environmental and Social Consequence

Dioscorea polystachya is a fast-growing vine that has the ability to rapidly invade undisturbed habitats, such as riparian forests. It spreads prolifically by asexual reproduction via bulbils. In the U.S., it reduces native species richness and abundance by outcompeting and eliminating native plant species. It thickly blankets adjacent vegetation, and excludes light from understory vegetation. It can break branches of large trees and shrubs. It is able to completely cover the ground so that the growth of native herbaceous ground cover is prevented (ISSG, 2009).

Uncertainty

Although it is listed on one nursery website there is no definite information to confirm that Dioscorea polystachya is being grown in gardens in Canada or if it is present. An online search found no other nurseries or seed suppliers that offer the species for sale in Canada.

Conclusion

Based on the outcome of this pest risk assessment, Dioscorea polystachya is likely to become weedy or invasive in parts of Canada, including southern and coastal British Columbia, southern Ontario, southern Quebec and parts of the Maritime Provinces. This plant should be considered for regulation under Canada's Plant Protection Act. Regulation under the Seeds Act may also be warranted, although seed is not currently considered to be a likely pathway for this species.

Technical Issues for Consideration

The very large number of species in Dioscorea complicates identification. There is one native species of this genus in southern Ontario, Dioscorea villosa L. This species is distinguishable from Dioscorea polystachya, with care and training.

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