RMD-13-04: Consolidated Pest Risk Management Document for pest plants regulated by Canada
Appendix 14A: Pest Risk Assessment Summary for Solanum elaeagnifolium (silverleaf nightshade)

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Identity of Organism

Name: Solanum elaeagnifolium (Solanaceae; USDA-ARS, 2009)

Synonyms: None listed (e.g. CAB International, 2007; USDA-ARS, 2009; USDA-NRCS, 2009).

English common names: Bitter apple, bitterleaf nightshade, bull nettle, prairie-berry, silverleaf bitter-apple, silverleaf nightshade, silverleaf-nettle, tomato weed, white horse-nettle (EPPO, 2007; USDA-ARS, 2009).

French common names: Morelle jaune (EPPO, 2007)

Description: Solanum elaeagnifolium is an erect, deep-rooted, shrub-like, perennial herb. Plants are multi-stemmed and highly branched; reaching a height of up to 80 cm. Leaves are alternate, stalked, and lance-shaped with wavy or scalloped edges. Stems and leaves are densely covered with short, fine, star-shaped hairs that give the plant a silvery-white appearance. Stems and main veins of leaves also have numerous slender, yellow to red prickles. Flowers are star-shaped and bright blue to purple/violet or occasionally white, with 5 fused petals and 5 prominent yellow anthers. Flowers give rise to clusters of smooth, globular berries that change in colour as they mature from green with stripes to mottled yellow and orange or brownish when ripe. Each berry contains 60-120 seeds that are flat, light, and closely resemble those of tomatoes (Parsons and Cuthbertson, 2001; CAB International, 2007; EPPO, 2007; Mekki, 2007).

Solanum elaeagnifolium is a weed of crops, pastures, and disturbed areas in its native and introduced ranges worldwide. All parts of the plant, but particularly berries, are poisonous to livestock. Historically, the berries and seeds were used by aboriginal peoples in the south-western U.S. in the preparation of food and clothing (Boyd et al., 1984). More recently, the fruit has been identified as a rich source of steroidal alkaloids (e.g. solasodine), used in the synthesis of contraceptive and corticosteroid drugs. These have been commercially extracted in India and Argentina (Maiti and Mathew, 1967; Parsons and Cuthbertson, 2001; CAB International, 2007).

Organism Status

Solanum elaeagnifolium is not reported to occur in the Canadian flora (Scoggan, 1979; Kartesz, 1999; CFIA, 2008), and no evidence was found that it is cultivated in Canada. Based on this information, Solanum elaeagnifolium is considered absent with no pest records in the PRA area.

Current Regulatory Status

Solanum elaeagnifolium is not currently regulated as a pest in Canada, either federally or provincially (although Solanum spp. are regulated in Manitoba). It is not a Federal Noxious Weed in the U.S., but it is regulated as a noxious weed and/or noxious weed seed in 19 U.S. states (Alabama, Arkansas, Arizona, California, Colorado, Florida, Georgia, Hawaii, Idaho, Kansas, Louisiana, Michigan, New Mexico, Nevada, Oklahoma, Oregon, South Carolina, Texas, and Washington) (Rice, 1997-2008; USDA-ARS, 2009; USDA-NRCS, 2009). Worldwide, it is a quarantine pest in Belarus, Russia, and Ukraine, and is controlled under noxious weed legislation in Australia and South Africa (EPPO, 2007). In Europe, it is included on the European and Mediterranean Plant Protection Organization (EPPO) list of species recommended for regulation as quarantine pests by member countries (EPPO, 2009).

Probability of Entry

The most likely pathway of entry of Solanum elaeagnifolium into the PRA area is unintentional introduction as a contaminant in commodities or conveyances. It has been distributed around the world in contaminated fodder and crop seed, and may also be spread by livestock and manure, agricultural machinery, vehicles, bulldozers and other earth-moving equipment (Boyd et al., 1984; Parsons and Cuthbertson, 2001; CAB International, 2007; EPPO, 2007; Mekki, 2007). Soil, sand, and ornamental plants can also be contaminated by fragments of roots or seeds (EPPO, 2007).

Table 1 : Summary of Pathways for Solanum elaeagnifolium (silverleaf nightshade)
Type of pathways Specific pathways
Natural dispersal

Natural spread of Solanum elaeagnifolium occurs by seed and vegetatively from cut root sections. Seeds are fleshy berries adapted to dispersal by birds and animals. Studies have shown seed to be viable after passing through the digestive tracts of animals (Parsons and Cuthbertson, 2001). The fruit can also float, and be dispersed along rivers and streams (Boyd and Murray, 1982; EPPO, 2007) and dried berries can be dispersed by strong winds in the winter (Parsons and Cuthbertson, 2001).

This is a possible pathway of entry into Canada, as there are populations in the U.S. approaching the Canadian border (e.g. Washington). Natural spread by vegetative fragments would be local, but spread of seeds by birds, animals, wind, or water could cover greater distances.

Intentional introduction

Solanum elaeagnifolium is not generally cultivated as an ornamental (Bailey and Bailey, 1976; Isaacson and Allen, 2007). Solanum elaeagnifolium has some limited use as fodder in South Africa (CAB International, 2007), and the steroidal alkaloid solasodine has been commercially extracted in India and Argentina (EPPO, 2007). However, it is unclear to what extent (if any) the plant is cultivated in this regard.

This pathway is considered unlikely at this time.

Unintentional introduction

Solanum elaeagnifolium is a weed of crops and pastures, and has been distributed around the world in contaminated fodder and crop seed (Parsons and Cuthbertson, 2001). It may also be spread by livestock and manure, agricultural machinery, vehicles, bulldozers and other earth-moving equipment, as well as in soil, sand, and ornamental plant material (EPPO, 2007; Boyd et al., 1984).

This is considered the most likely pathway of entry into Canada.

Probability of Establishment

Solanum elaeagnifolium is native to the south-western U.S. and north-eastern Mexico. It is sometimes considered native to Argentina, although the nature of the herbivorous insect fauna suggests this is a secondary distribution (Boyd et al., 1984; CAB International, 2007; EPPO, 2007). It has been introduced elsewhere in North and South America, and is now widespread in the U.S. in all but the Great Lakes and New England regions (Figure 1; Kartesz, 1999; CAB International, 2007; USDA-NRCS, 2009). It is also introduced in other parts of the world including Australia, India, South Africa and around the Mediterranean basin (Parsons and Cuthbertson, 2001; CAB International, 2007; EPPO, 2007; GBIF, 2008). Most of its range falls in NAPPFAST zones 8-9 and above (www.nappfast.org), but occurrences in the U.S. suggest it can survive in zone 6 and probably 5 (Figure 2).

Figure 1: Current distribution of Solanum elaeagnifolium (silverleaf nightshade) in North America

Figure 1. Description follows.
Description for Figure 1:

This map shows the current distribution of in North America indicating its presence through the use of the colour green. Two thirds of the United States is covered in green predominantly the lower western half, none of the states along the north eastern coast contain green except for Maryland.

Source: USDA-NRCS, 2009.

Solanum elaeagnifolium is adapted to a wide range of habitats but appears mostly in warm-temperate regions in areas of relatively low annual rainfall (250-600 m). It thrives on disturbed land and is not confined to any particular soil type. Infestations are more serious in dryland situations, but irrigated crops are also prone to invasion (Parsons and Cuthbertson, 2001; CAB International, 2007). In addition to cultivated and agricultural land, it occurs along roadsides, railways, riverbanks, and canal-sides, and in rangeland, livestock corrals, construction sites, and wastelands (CAB International, 2007; EPPO, 2007; Mekki, 2007).

Based on its native distribution and available records, it appears that Solanum elaeagnifolium may find climatic and ecological conditions suitable for establishment and spread in the PRA area, in the parts of British Columbia, southern Ontario and the Maritimes which comprise zones 6-9 and possibly also zone 5.

Figure 2-1: Potential range of Solanum elaeagnifolium (silverleaf nightshade)

NAPPFAST zones 6-9. Description follows.
Description for Figure 2-1:

This image shows the potential range of Solanum elaeagnifolium (silverleaf nightshade) in Canada and northern America through the use of a map. Red is used to indicate the regions in which Solanum elaeagnifolium could survive in according to the Canadian Plant Hardiness Zones map, in this case NAPPFAST Hardiness Zones 6-9. In Canada, this includesSolanum elaeagnifolium is likely to become weedy or invasive in parts of Canada, including southern and coastal British Columbia, southern Ontario, and the Maritimes. Most of the red can be found in the U.S., this includes the parts of the east and west coasts as well as areas located near the Great Lakes extending downwards beyond the image.

NAPPFAST zones 6-9.

Figure 2-2: Potential range of Solanum elaeagnifolium (silverleaf nightshade)

NAPPFAST zones 5-9. Description follows.
Description for Figure 2-2:

This image shows the potential range of Solanum elaeagnifolium in Canada and northern America through the use of a map. Red is used to indicate the regions in which Solanum elaeagnifolium could survive in according to the Canadian Plant Hardiness Zones map, in this case NAPPFAST Hardiness Zones 5-9. In Canada, the potential range of Solanum elaeagnifolium includes parts of Atlantic Canada, southern Ontario and coastal and southern BC. Most of the red can be found in the U.S., this includes the greater parts of the east and west coasts extending inwards reaching most of the central states as well as areas located near the Great Lakes extending downwards beyond the image.

NAPPFAST zones 5-9.

Probability of Spread

Natural spread of Solanum elaeagnifolium occurs both by seed, and vegetatively from cut root sections. Flowers are cross-pollinated by insects, primarily bees (Hardin et al., 1972; EPPO, 2007), and produce clusters of fleshy berries that may be dispersed by birds, animals, wind and water. A single plant generally produces 40-60 fruits per growing season (up to 200), each containing 60-120 seeds (CAB International, 2007; EPPO, 2007). Dense populations are capable of producing 250 million seeds/ha (Boyd and Murray, 1982). Seeds are highly viable and may also last up to 10 years in soil (EPPO, 2007; Mekki, 2007). Plants are also able to regenerate vegetatively as early as ten days after germination (EPPO 2007). Established plants have extensive root systems, and new shoots are produced from the lateral roots each spring from as deep as 50 cm. All parts of the root are capable of forming shoot buds (Parsons and Cuthbertson 2001); root fragments only 1 cm long retain the ability to sprout, and remain viable for up to 15 months (Boyd et al. 1984; Parsons and Cuthbertson 2001; EPPO 2007). Despite this variety of natural dispersal mechanisms, most long-distance spread has been attributed to human activity.

Potential Economic Consequences

Solanum elaeagnifolium is an agricultural weed that competes for moisture and nutrients with a variety of crops in both dryland and irrigated conditions. Several crops are affected worldwide, the most important of which are cereals (wheat, sorghum, maize), alfalfa, and cotton (Parsons and Cuthbertson, 2001; CAB International, 2007). The most serious crop losses have been recorded in alfalfa (in Australia, South Africa, and U.S.), cotton, sorghum, maize and groundnut (in Morocco, U.S.), wheat (in Australia, U.S.) and cultivated pastures (in Australia, Greece, Morocco, U.S.) (CAB International, 2007). Other crops affected include many vegetables (e.g. potato, asparagus, tomato), grapes, and some fruit trees (e.g. peaches). In addition to yield reductions, presence of the weed in harvested products reduces their quality and sale (CAB International, 2007).

Potential Environmental and Social Consequences

Solanum elaeagnifolium is primarily an agricultural weed that invades croplands, pastures, and disturbed and marginal areas such as roadsides. No reports of environmental impacts or effects on natural areas were found, although it may replace natural vegetation in overgrazed rangeland (EPPO, 2007; Mekki, 2007). Social impacts may result from the loss of value of agricultural land infested with the weed; in Morocco the value of infested fields reportedly decreased by 25%, while in the U.S. entire farms have been abandoned because of the weed (Gmira et al., 1998; EPPO, 2007).

Uncertainty

Further investigation of the distribution of Solanum elaeagnifolium in the U.S. and Europe could help refine the estimated potential range in Canada. No other significant areas of uncertainty were identified.

Conclusion

Based on the outcome of this risk assessment, Solanum elaeagnifolium has the potential to become weedy or invasive in areas in which it can establish. Parts of British Columbia, southern Ontario and the Atlantic provinces are considered to be the areas most at risk. This plant should be considered for regulation according to Canada's Plant Protection Act and Seeds Act.

Technical Issues for Consideration

Various other species of the genus have reportedly been confused with Solanum elaeagnifolium in the field in Australia, South Africa and the U.S. However, it is not generally considered difficult to distinguish or detect as a commodity contaminant (CAB International, 2007).

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