Solanum elaeagnifolium
(silverleaf nightshade)

Pictures

Picture	Title	Caption	Copyright
Title Solanum elaeagnifolium plant in flower Caption Copyright T. Olckers/ARC-PPRI	Solanum elaeagnifolium plant in flower		T. Olckers/ARC-PPRI

Identity

Preferred Scientific Name

• Solanum elaeagnifolium Cavanilles

Preferred Common Name

• silverleaf nightshade

Other Scientific Names

• Solanum dealbatum Lindl.
• Solanum flavidum Torr.
• Solanum leprosum Ortega
• Solanum obtusifolium Willd.

International Common Names

• English: bull nettle; prairie berry nightshade; silverleaf nettle; silver-leaf nightshade
• Spanish: meloncillo del campo (Argentina); quillo-quillo (Argentina); revienta caballo (Argentina); tomatillo (Chile); trompillo (Honduras); white horse-nettle

Local Common Names

• Australia: tomato weed; white horsenettle
• Brazil: arrebenta-cavalo; melãozinho-do-campo
• Finland: tähtikoiso
• Germany: Nachtschatten, Ölweidenblättriger
• India: white horsenettle
• Morocco: chouika; chouk jmel; chouka assafra; hassika matechat jmel
• South Africa: bitter apple; bitterappel; bitterleaf nightshade; bloubos; satansbos; silverleaf bitter apple; silwerblaarbitterappel
• Sweden: silverskatta
• USA: silver-leaf nettle
• USA/California: silver-leaf nettle

EPPO code

• SOLEL (Solanum elaeagnifolium)

Summary of Invasiveness

S. elaeagnifolium is a deep-rooted summer-growing perennial plant, native to the Americas, but now widely naturalized beyond its native range in extra-tropical regions. It is considered a tenacious weed in many arid to semi-arid places including India, Australia, South Africa, the Pacific Islands, and the USA (Holm et al., 1979; Wagner et al., 1999; Randall, 2012; USDA-ARS, 2014). It is known to be invasive in Cuba (Oviedo-Prieto et al., 2012) and Hawaii (PIER, 2014), a principal weed in India (Holm et al., 1979), and an agricultural weed in Java (Randall, 2012). It has been declared a noxious weed in the U.S. states of Arkansas, California, Idaho, Nevada, and Washington, and an “A” designated weed for quarantine in Oregon and Washington (USDA-NRCS, 2014). The species competes with crops, interferes with livestock, acts as a host for insects and plant diseases, and spreads by forming dense colonies from its extensive root system as well as by propagation of seeds (Boyd et al., 1984; Wagner et al., 1999; EPPO, 2007; PIER, 2014). The species is difficult to control without chemicals (UC Davis Weed Research and Information Center, 2013) and it is essential to keep it out of uncontaminated areas (EPPO, 2007). The species is known to be toxic to cattle, causing damage to intestinal tract and nervous systems and, in severe cases, can cause hallucinations, paralysis, and death (Mas and Lugo-Torres, 2013).

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Plantae
•         Phylum: Spermatophyta
•             Subphylum: Angiospermae
•                 Class: Dicotyledonae
•                     Order: Solanales
•                         Family: Solanaceae
•                             Genus: Solanum
•                                 Species: Solanum elaeagnifolium

Notes on Taxonomy and Nomenclature

Solanaceae, the Nightshade family, consists of 90 genera and 3000-4000 species with great variation in habit and distribution on all continents except Antarctica, with the majority of species diversity in Central and South America (PBI Solanum Project, 2014).

Solanum is one of the largest genera of vascular plants with about 1000-1500 species, 1000 of which are speculated to be of American origin (Hunziker, 1979). Taxonomy of the genus and its seven subgenera have undergone many revisions, but the overall genus consists of herbs, shrubs, trees, or herbaceous or woody vines, usually with spines or prickles, glabrous or pubescent with simple or stellate hairs (Acevedo-Rodriguez, 1996). While the etymology of the genus’ scientific name is unclear, it may be derived from the Latin word “sol”, meaning "sun," referring to its affinity for sunlight, or from the Latin word “solare”, meaning "to soothe”, the Latin word “solamen”, meaning "a comfort", or the Akkadian word “sululu”, meaning “happy”, in reference to the narcotic effects of some Solanum species after ingestion (Smith, 1971; Wiart, 2006; Quattrocchi, 2012; New Zealand Plant Conservation Network, 2014).

The name S. elaeagnifolium is universally accepted for this weed. The plant displays considerable morphological variation in the Americas which has confused its taxonomic status. Although Morton (1976) proposed that the Argentinian form was a separate subspecies, Symon (1981) concluded that this form also occurs in North America and is thus a natural variant of the same species. Indeed, variation within populations may include the degree of spininess, growth habit, petal colour, and the shape, size and lobing of the leaves. In Australia and South Africa this variability is considered to be the result of multiple introductions, rather than hybridization with native Solanum species (Stoltsz, 1994; Heap et al., 1997). The weed's facility for clonal reproduction permits the coexistence of different variants, but classification at the varietal level should be avoided unless genetic differences are confirmed (Stoltsz, 1994).

Description

Perennial herbs up to 50 cm tall, vegetative growth usually annual, erect, branched above, usually armed with straight, fine, reddish prickles 2-5 mm long, usually on stems, occasionally on petioles, leaves, and calyx, all parts densely and closely tomentose with stellate hairs, general aspect silvery green, rarely reddish brown, forming colonies from underground root system. Leaves simple, alternate, lower leaves oblong-lanceolate, up to 10 cm long and 4 cm wide, margins sinuate-undulate, apex acute or obtuse, base rounded or cuneate, upper leaves smaller, oblong, entire. Flowers perfect, actinomorphic, few in racemose cymes, peduncle up to 1 cm long, pedicels ca. 1 cm long at anthesis, elongating to 2-3 cm long in fruit; calyx tube up to 5 mm long, 5-ribbed by the principal veins, the lobes subulate; corolla blue, rotate-stellate, 2-3 cm in diameter, the lobes divided ca. 1/2 their length; stamens inserted near base of corolla tube; filaments 3-4 mm long; anthers yellow, slender, tapered upward, conspicuous, erect, not coherent, 5-8 mm long, opening by apical pores; ovary pubescent toward summit; style 10-15 mm long; stigma terminal. Berries at first marbled green, later yellow to finally orangish brown, mucilaginous, globose, 0.8-1.4 cm in diameter, calyx covering base of fruit. Seeds pale brown, discoid, flattened, ca. 3 mm long, smooth. [Wagner et al., 2014]

Plant Type

Distribution

S. elaeagnifolium is native to north-east Mexico and the south-west USA (Goeden, 1971; Boyd et al., 1984; Wapshere, 1988). Although it is also thought to be indigenous to Argentina, the nature of the insect herbivore faunas in this country suggests that this distribution is secondary (EPPO, 2007). USDA-NRCS (2014) reports the species as native to all the North American states listed by the source, although a note in the USDA-ARS (2014) database says ‘probably not native to North America’, and EPPO (2007) quotes Goeden (1971) as saying that in California it was introduced in 1890.

The species is now widely naturalized and a tenacious weed. It is invasive to Hawaii (PIER, 2014), and is naturalized on O`ahu, Moloka`i, and Maui Islands (Wagner et al., 2014). Other countries of introduction have included Australia, India, North Africa, South America, Mediterranean countries and southern Africa (Wassermann et al., 1988). The species was newly recorded in southern Taiwan and the Penghu Islands in 2002 and reported but unconfirmed in Turkey (EPPO, 2007). While the species has been recorded as present or even naturalized in parts of South America (USDA-ARS, 2014), the species is not included in Funk et al.’s (2007) flora of the Guiana shield, Broome et al.’s (2007) work on the eastern Caribbean, or in Forzza et al.’s (2010) flora of Brazil.

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

S. elaeagnifolium is considered to be native to the Americas, although it may have been introduced to the northern and eastern parts of North America (EPPO, 2007; USDA-ARS, 2014). The species has spread primarily as a seed contaminant in soil and crops. Spanish or Portuguese colonists may have been instrumental in spreading the species across the Americas, and it is thought to have been introduced to California by contaminated railway cars (Boyd et al., 1984). The species was first recorded for Australia in 1901, for Israel during the 1956 war, and to Morocco in 1958 through contaminated crop seeds (EPPO, 2007). In South Africa, the species is thought to have been imported as either a contaminant of pig fodder around 1905 or as a hay contaminant during the 1940s or 1950s, before it was declared a weed in 1966 (EPPO, 2007). The species is thought to have been introduced from Mexico to the Philippines sometime during the Spanish colonial period through the Manila-Acapulco galleon trade (1585-1615), and from there to China and the rest of Asia (PBI Solanum Project, 2014). Date of introduction to the West Indies is uncertain but may have been relatively recent. Smithsonian Herbarium specimens of this species were collected in Cuba in 1919, Curaçao  in the 1950s, and Puerto Rico in the 1960s; for the West Indies, as of 2007 EPPO only reported its presence in Puerto Rico (EPPO, 2007). It has now also been reported for Tortola, Virgin Gorda, and the Bahamas (Acevedo-Rodriguez and Strong, 2012).

Risk of Introduction

S. elaeagnifolium has the potential to invade ecosystems and out-compete native flora by forming dense colonies. It reproduces both by seed and vegetatively, with rhizomes and root fragments capable of generating new plants. The species is listed as an agricultural weed, casual alien, naturalised, and weed in the Global Compendium of Weeds (Randall, 2012). It has been recorded as a cultivation escape in fields in Baluchistan and Sind, Pakistan (Flora of Pakistan, 2014) and is an agricultural weed in the Pacific (Randall, 2012). In India the species is classified as a ‘serious weed’, the highest rank in Holm et al.’s (1979) geographical atlas of world weeds. It is also known to be a weed in Argentina, Chile and Mexico (Holm et al., 1979). Considering the species is known to be invasive to Hawaii and Cuba, and a declared noxious weed in the United States, Australia, and South Africa, and considering its invasive traits and likeliness of further spread, the risk of introduction for this species is high.

Habitat

S. elaeagnifolium is adapted to a wide range of habitats, but appears mostly in areas of relatively low annual rainfall (300-500 mm) (Parsons, 1981; Heap et al., 1997). The weed thrives on disturbed land and, in addition to crop lands, areas particularly prone to invasion include roads, water furrows and rivers, and livestock corrals (Wassermann et al., 1988). Invasion of rangeland has also been recorded in South Africa where it is aggravated by trampling caused by livestock. In Paraguay, the species occurs in gallery and low forest zones and moist savanna (Paraguay Checklist, 2014). In Guatemala, the species has been found in marshes and roadsides (Flora Mesoamericana, 2014). In Java, the species is known as an agricultural weed of sugarcane and cultivated fields (Randall, 2012). The species is known to occur in Pakistan at altitudes up to 1800 m (Flora of Pakistan, 2014).  
 

Habitat List

Hosts/Species Affected

Several cultivated crops are affected by this weed worldwide (see Economic Impact text and Host Plant table), the most important of which are cereal crops (wheat, sorghum, maize), lucerne and cotton. Infestations are more serious in dryland situations, although irrigated croplands are also very prone to invasion. S. elaeagnifolium is known to be an agricultural weed in sugarcane fields of Java (Randall, 2012).
 

Host Plants and Other Plants Affected

Symptoms

Invasions of croplands and paddocks by S. elaeagnifolium are most conspicuous during midsummer when the plants are flowering. In such infestations, the plants are easily identified by the abundance of blue or purple flowers and the orange-yellow mature berries that appear later in the season. There are no symptoms on the tissues of the crop species themselves, although yield losses resulting from competition for moisture and nutrients are obvious.
 

Biology and Ecology

Genetics

The chromosome count for this species is 2n = 24, 72 (Wagner et al, 1999).

Physiology and Phenology

S. elaeagnifolium is a deep-rooted, shrub-like, perennial plant in which the aerial growth dies back in late autumn, surviving off its rootstocks during the winter. The plants have very extensive, spreading root systems that can penetrate to depths in excess of 3 m. New shoots develop from adventitious buds on the roots, allowing very effective vegetative propagation during spring and after cultivation. The plants produce bright blue to purple flowers (although white flowers are a rare occurrence) during spring to autumn. Although the plants die back in winter, ripe fruit are retained on dead branches and may be dispersed by wind.

Single plants can produce up to 200 berries per growing season, and thus thousands of seeds. The longevity and high viability of the seeds ensure survival through long periods of unfavourable growing conditions. Seeds germinate in autumn and the young plants develop an extensive root system during the first months (De Beer, 1985). Germination is advanced by alternating temperatures and favourable moisture conditions, notably heavy rains.

Environmental Requirements

S. elaeagnifolium is found in many vegetation zones including humid lowland evergreen tropics, dry Acacia-cactus thorn-scrub, and in Pinus-Quercus forests (PBI Solanum Project, 2014). It can grow in areas with a hot dry summer and a cool wet winter, and can apparently withstand low temperatures of –23 to –18°C while requiring high temperatures of 20-34°C for germination and growth (EPPO, 2007). In the Mediterranean region, S.elaeagnifolium tolerates steppe and mild climates with relatively high summer temperatures and low annual rainfall (250–600 mm) (EPPO, 2007). In Paraguay the species can be found in moist savannah climates (Paraguay Checklist, 2014). The species appears to prefer loamy, droughty soils, but is found on virtually all soil types including nutrient-poor sandy soil, although it cannot tolerate deep sands (USDA-NRCS, 2003; EPPO, 2007). Sandy soils with low organic matter have supported the heaviest infestations in Australia (Heap et al., 1997). Full sunlight is most favourable for growth, and shading reduces plant vigour (EPPO, 2007), although the species still grows in shade and can become abundant under trees and beside farm buildings, even creating thickets (USDA-NRCS, 2003; PBI Solanum Project, 2014). Established plants are sensitive to frost and waterlogging but are tolerant to saline conditions and highly resistant to drought (EPPO, 2007). 

Climate

Latitude/Altitude Ranges

Air Temperature

Rainfall

Rainfall Regime

Soil Tolerances

Soil drainage

• free

Soil reaction

• acid
• alkaline
• neutral

Soil texture

• heavy
• light
• medium

Special soil tolerances

• infertile
• saline
• shallow

Natural enemies

Notes on Natural Enemies

Several species of herbivorous insect constitute natural enemies of S. elaeagnifolium in the plant's countries of origin (Goeden, 1971), some of which have been considered as biological control agents (Neser, 1984; Wapshere, 1988; Olckers and Zimmermann, 1991). The insect herbivore faunas in Mexico and south-western USA are more diverse, and include more specialists (species with narrow host ranges) than those in Argentina, confirming suspicions that the plant is a relatively recent introduction into Argentina. By contrast, S. elaeagnifolium is not subject to significant natural enemy pressure in its countries of introduction, as reported in South Africa (Hill et al., 1993), suggesting that it is a good target for biological control.

Several countries, notably South Africa and Australia, have investigated the feasibility of biocontrol, but only South Africa has imported, tested and released any agents so far. Two leaf-feeding beetles, Leptinotarsa texana and Leptinotarsa defecta, have become established in South Africa following deliberate releases (see Control). The leaf-galling nematode Ditylenchus phyllobius (= Orrina phyllobia) has become established in India (Wassermann, 1988; Olckers and Zimmermann, 1991), and was probably originally introduced together with the plant. D. phyllobius was considered for release in South Africa, but was rejected because of doubts about its host specificity (Olckers and Zimmermann, 1991).
 

Means of Movement and Dispersal

Natural dispersal (non-biotic)

Long-distance spread can occur both vegetatively, from cut root sections, and via seeds. Dispersal is aggravated by agricultural practices (contaminated vehicles and implements) and via agricultural produce, notably seeds that are harvested with certain crops.

Seedborne spread

Individual plants usually bear 40-60 fruits, each containing 60-120 seeds similar in size and shape to those of tomatoes, that can remain viable for up to 10 years in soil (EPPO, 2007), although recent research suggests it may be shorter (Kidston et al., 2007). The species has been reportedly capable of producing up to 200 berries per season, resulting in a prolific 1500-7200 seeds per plant (EPPO, 2007). These seeds are spread by birds, water and livestock, especially sheep, as digestion may enhance germination (Boyd et al., 1984).

Seeds of the weed may be transported via seeds of harvested crops, notably grain and fodder crops, that are contaminated.

Agricultural practices

Vehicles and implements used in agriculture (also bulldozers and earth-moving equipment) can spread the weed by transporting both seeds and sections of root. Also, seeds harvested together with crops, notably fodder crops such as lucerne, present a real threat in initiating new infestations.

The ability of the species to reproduce vegetatively has contributed to its wide distribution, particularly surrounding cultivated areas where tillage helps to fragment existing root systems and aid its spread (Kidston et al., 2007; UC Davis WRIC, 2013). Rhizome fragments as small as 0.5 cm long are capable of regenerating, and sections of taproot can remain viable for up to 15 months (Boyd et al., 1984; EPPO, 2007). 

Movement in trade

Crop species, notably grain and fodder crops, that are contaminated with the weed are a major source of infestation. Indeed, accidental importation with pig fodder in 1904 was the presumed mode of entry into South Africa and the source of later infestation (Wassermann et al., 1988). Infestations in Australia have similarly been linked to importations of contaminated hay from North America during the 1914 drought (Heap et al., 1997). Contaminated ballast and bedding used in railroad cattle cars led to the introduction of the weed into California in 1890 (Goeden, 1971).
 

Pathway Causes

Pathway Vectors

Plant Trade

Wood Packaging

Impact Summary

Economic Impact

S. elaeagnifolium competes for moisture and plant nutrients with a variety of crops under both dryland and irrigated conditions. Marginal, dryland cropping areas are particularly at risk. The most serious crop losses have been recorded in lucerne (in Australia, South Africa and the USA); cotton, sorghum, maize and groundnut (Morocco, USA); wheat (Australia, Greece); and cultivated pastures (Australia, Greece, Morocco, USA) (Cuthbertson, 1976; Molnar and McKenzie, 1976; Robinson et al., 1978; Abernathy and Keeling, 1979; Boyd et al., 1984; Tanji et al., 1984; Wassermann et al., 1988; Eleftherohorinos et al., 1993). In Australia, wheat production losses have varied from 12-50% (Cuthbertson, 1976), while sorghum and cotton losses under optimal moisture regimes have varied from 4-10% and 5-14%, respectively, in the USA (Robinson et al., 1978), and have even reached 75% in cotton grown under semi-arid conditions in the USA (Abernathy and Keeling, 1979). Pasture establishment is also delayed and pasture production is reduced by the weed. Other crops that have been affected include potato, peach, asparagus and Bermuda grass in the USA, and cucumber, grape, olive and tomato in Greece (Eleftherohorinos et al., 1993). The weed has also displayed allelopathic effects on several crops (for example, asparagus in Greece). In addition to yield reductions, the presence of the weed in harvested products reduces their quality as well as their sale. The sale of agricultural products contaminated with the weed is prohibited in South Africa (Wassermann et al., 1988).

S. elaeagnifolium is perennial, very difficult to control, and often disrupts tillage and harvesting practices (Wassermann et al., 1988). Severe infestations have limited the types of crop that could be successfully cultivated in the USA (Davis et al., 1945), where in some cases farms were abandoned because of ineffective production (Parsons, 1981). The implications of this are less effective land usage, reduced monetary returns, and increased production costs arising from control operations. In the USA the weed has been reported as a vector of Lettuce chlorosis virus (California) (McLain et al., 1998) and as an occasional secondary host of several insect crop pests, the most important of which are the Colorado potato beetle (Texas) (Hare, 1990) and the pepper weevil (Florida) (Patrock and Schuster, 1992).

The berries of S. elaeagnifolium are also toxic to livestock and may thus reduce the value of agricultural land (Wassermann et al., 1988). Mature berries are reportedly more toxic than immature ones (Burrows et al., 1981). In the USA, animals that ingested 0.1-0.3% of their body mass in berries displayed mild symptoms of poisoning (Dollahite and Allen, 1960), while in Australia sheep mortalities were reported (Molnar and McKenzie, 1976). Cattle are reported to be more susceptible to toxicity than sheep, while goats are apparently unaffected (Parsons, 1981; Wassermann et al., 1988). In Argentina there have been cases of horses being poisoned by the weed (Anon., 1980). Symptoms of poisoning included salivation, nasal discharge, respiratory complications, bloating, trembling and diarrhoea (Parsons, 1981).

Risk and Impact Factors

• Invasive in its native range
• Proved invasive outside its native range
• Has a broad native range
• Abundant in its native range
• Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
• Pioneering in disturbed areas
• Tolerant of shade
• Highly mobile locally
• Benefits from human association (i.e. it is a human commensal)
• Has high reproductive potential
• Has propagules that can remain viable for more than one year
• Reproduces asexually

• Damaged ecosystem services
• Monoculture formation
• Negatively impacts agriculture
• Negatively impacts animal health
• Negatively impacts tourism
• Reduced amenity values
• Negatively impacts trade/international relations

• Competition - monopolizing resources
• Pest and disease transmission
• Produces spines, thorns or burrs

• Highly likely to be transported internationally accidentally
• Difficult to identify/detect as a commodity contaminant
• Difficult to identify/detect in the field
• Difficult/costly to control

Uses

Although the disadvantages of S. elaeagnifolium are overriding, it does have a few useful attributes. Despite confirmed toxicity to livestock in some situations (De Beer, 1985), the weed has some grazing value in South Africa (Wassermann et al., 1988). Chemical analyses of the plants and seeds indicated a relatively high crude protein content. Cattle and wild antelope have been reported to browse on the weed during spring and early summer, when natural grasses are unpalatable, with no ill effects (Wassermann et al., 1988). However, the plants become less palatable with the onset of flowering, possibly because of an increase in thorniness. New growth is particularly favoured, and cutting back mature plants restores palatability. S. elaeagnifolium has also been pelleted and successfully fed to animals in South Africa. Although the weed may be regarded as a useful drought-resistant fodder in some areas of South Africa, this is not the case in the USA (Dollahite and Allen, 1960) and Australia (Molnar and McKenzie, 1976), where stock losses through poisoning have been reported.

Solanum species are also a rich source of steroidal alkaloids that are used in the synthesis of contraceptive and corticosteroid drugs. Solasodine, the most important of these precursors, has been commercially extracted from the berries of S. elaeagnifolium in India (Maiti and Mathew, 1967) and Argentina. Solasodine yields amount to 3.2% of berry dry weight, making S. elaeagnifolium the most promising source of the several Solanum species investigated (Heap et al., 1997).

Aerial parts of the plant have been used by American Indians of the southwest USA in food and clothing preparation; the Pima tribe reportedly used the berries and seeds when making cheese, and the Kiowas used the seed in a mixture for tanning hides (Boyd et al., 1984). 

Uses List

Animal feed, fodder, forage

• Forage

Materials

• Poisonous to mammals

Medicinal, pharmaceutical

• Source of medicine/pharmaceutical
• Traditional/folklore

Detection and Inspection

The species is difficult to detect, as its seeds are small and numerous, and rhizome fragments as small as 0.5 cm have the ability to regenerate and can remain viable for up to 15 months (Boyd et al, 1984; EPPO, 2007; UC Davis Weed Research and Information Center, 2013). Weed infestations are most conspicuous during midsummer when the plants are flowering. In susceptible areas, regular paddock inspections during this time, and vigilance during harvesting, increase the chances of detecting new weed patches which should then be monitored (Heap et al., 1997). Importations of grain and fodder crops from contaminated areas should be subject to inspections aimed at detecting seed contaminants, while livestock should be subject to a quarantine period (around 14 days) to allow ingested seed to be flushed out. Vehicles, agricultural implements and earth-moving machinery that exit contaminated areas should also be inspected for seeds and root fragments.
 

Similarities to Other Species/Conditions

S. elaeagnifolium has been confused with S. coactiliferum, S. esuriale and S. karsensis in Australia (Heap et al., 1997), S. carolinense in Texas, USA (Gorrell et al., 1981), and S. panduriforme in South Africa (Wassermann et al., 1988). Solanum dimidiatum (western horse-nettle) is somewhat similar but lacks the velvety appearance, and Crotoncapitatus (wooly croton, doveweed, goatweed), which also resembles S. elaeagnifolium, is never armed, and is very intolerant of shade (USDA-NRCS, 2003). In the EPPO region, S. elaeagnifolium is very distinct from other native or introduced Solanum spp. (EPPO, 2007).

Prevention and Control

Introduction

Several control methods have been investigated for S. elaeagnifolium, but none has proved conclusive in isolation. This is largely because of the weed's extensive and deep root system which contains large reserves that are rapidly translocated. However, combinations of different methods in integrated control programmes may be more successful. The key to effective control lies in sustaining pressure on the aerial parts of the weed and thereby eroding the root reserves. The tendency of the weed populations to die back in late autumn and subsist on their root reserves until spring suggests that they are more vulnerable to control programmes during spring and late autumn when the plants are building up their root reserves. Quarantine and exclusion are regarded as the most important control strategies in Australia (Heap et al., 1997).

Mechanical and Cultural Control

In the USA, appropriate management practices in arable land, including shading by crops, have controlled the weed within a 3-year period (Davis et al., 1945; De Beer, 1987). In winter crop situations (for example, small grains), regular tillage during the previous summer weakens the weed because of the lack of winter growth. In summer crops (for example, cotton, sorghum), where there is competition for water and light, dense stands of a crop have been able to suppress the weed, and regular tillage or clearing further depletes the weed's reserves and prevents fruit set. Cultural control tends to be more successful in irrigated croplands (for example, lucerne) where dense stands of the crop can be maintained. Mechanical control is difficult and inappropriate outside crop lands, although intensive browsing by stock can reduce fruit set (Wassermann et al., 1988). However, as recorded in Australia, mechanical control methods - notably slashing, cultivation (for example, deep ploughing) and grazing - result in temporary control of shoots but do not greatly damage the roots, and regrowth soon occurs (Heap et al., 1997). Cultural control methods may aggravate infestations, as new plants arise from transplanted fragments.

Grazing is a poor method of control, as cattle can only handle up to 25% of forage and the berries are known to be toxic to cattle (UC Davis Weed Research and Information Center, 2013). Grazing on fruiting plants is particularly discouraged, as 10% of seed that passes through the gut remains viable, and takes up to 2 weeks to be excreted (Kidston et al, 2007).

Mowing and cutting can provide 50-80% control of the species, as it can reduce infestation and seed production; however plants will develop rosettes below the mower blades (UC Davis Weed Research and Information Center, 2013). Tillage is more likely to increase the infestation than reduce it, as this will multiply the rhizome fragments within the soil and drag the pieces into clean areas.  Slashing is not an option, either, as the species quickly recovers and can form berries close to the ground (Kidston et al, 2007).

Chemical Control

Although a considerable amount of research on chemical control has been carried out worldwide, with varying degrees of success with different chemicals, S. elaeagnifolium is generally very difficult to control with herbicides. This is largely because of the weed's deep root system which is generally impervious to chemicals and ensures rapid recovery after spraying. This was experienced in South Africa (Wassermann et al., 1988) and Australia (Parsons, 1981; Heap et al., 1997), where a wide range of herbicides, including soil sterilants and non-selective chemicals, were used. Although herbicides can control seedlings and perennial plants during spot-spraying treatments, none is considered to provide effective and affordable control for large infestations of perennial plants. Effective chemical control is dependent on translocation and no root excretion (Heap et al., 1997).

In Greece, picloram proved to be the most effective herbicide, while glyphosate was inconsistent and triclopyr ineffective (Eleftherohorinos et al., 1993).

Picloram is often used in Australia to treat small infestations, but is unsuitable for large areas because of high costs and residual effects in the soil which are detrimental to broad-leaved crops and pasture species (Heap et al., 1997). In addition, 2,4-D as an ester or amine is used to suppress shoot growth and fruiting in Australia, but does not damage the roots sufficiently, while glyphosate has also failed to provide reliable control.

In South Africa, no herbicides are formally registered for this weed and recommendations for chemical control are restricted to the use of 2,4-D to inhibit growth and prevent fruit set (Wassermann et al., 1988). However, herbicides can augment cultural control methods and have been reputed to be more effective if applied during optimal moisture conditions and during the 'green berry' stage of the weed (Stubblefield and Sosebee, 1986). In addition, late-season applications (autumn) are more effective than those in early summer, which suggests more active translocation during this time.

Based on published papers and reports, 2,4-D, aminopyralid, dicamba, and picloram  generally provide more than 95% reduction, while glyphosate, mazapic, and triclopyr may also be effective (UC Davis Weed Research and Information Center, 2013).

Quarantine

Because the weed is very difficult to control conventionally, it is imperative that it is kept out of uncontaminated areas and that any isolated plants and small patches are treated as soon as they appear (Parsons, 1981). It is also imperative that new populations be excluded from cultivation, monitored, and subjected to repeated sprays when the plants recover. In Australia, livestock movements (notably sheep) account for most new infestations. Stock from infested areas should be quarantined for 14 days to allow all seed to pass through the digestive tract and thus prevent further spread (Heap et al., 1997). Unauthorized vehicles should also be kept out of infested properties, and vehicles and machinery should be thoroughly checked for berries and root fragments and cleaned on leaving infested paddocks (Anon., 1980).

Biological Control

In the USA, the native leaf-galling nematode Ditylenchus phyllobius [Orrina phyllobia], which is able to kill plants, has been used as an inoculant on weed infestations with some measure of success (Northam and Orr, 1982), but it is not host specific. Although several countries have considered classical biological control of S. elaeagnifolium, only South Africa has imported, tested and released biocontrol agents. This has been largely because of concerns about possible attacks on non-target Solanum species, notably cultivated crops (potato and aubergine), and native species. Indeed, several species have been rejected because of unacceptably broad host ranges during quarantine evaluations (Olckers and Zimmermann, 1991; Olckers et al., 1999). Biocontrol in South Africa currently relies on two leaf-feeding beetles, Leptinotarsa texana and Leptinotarsa defecta, both of which are established following releases in 1992. While L. defecta has remained localized and relatively scarce, with no obvious impact on the weed, L. texana has proved very effective. Indeed, L. texana reaches very high population densities and causes considerable damage to the weed, stunting vegetative growth and fruit-production capacity (Hoffmann et al., 1998; Olckers et al., 1999). Indications are that the beetles have the potential to contribute substantially to the control of the weed, particularly outside cropping areas where chemical and mechanical control methods are not cost-effective.

Verticillium dahlia is a fungus that has killed isolated plants, but is not considered a significant agent. EPPO (2007) suggests that biological control can only be considered as one component of an integrated management plan, to be used in conjunction with other specific management practices.

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Contributors

24/8/2014 Updated by:

Marianne Jennifer Datiles, Department of Botany-Smithsonian NMNH, Washington DC, USA

Pedro Acevedo-Rodríguez, Department of Botany-Smithsonian NMNH, Washington DC, USA

Distribution Maps