Solanum elaeagnifolium
(silverleaf nightshade)

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).

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.

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.

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).  
 

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).
 

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). 

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).
 

Animal feed, fodder, forage

• Forage

Materials

• Poisonous to mammals

Medicinal, pharmaceutical

• Source of medicine/pharmaceutical
• Traditional/folklore

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.
 

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).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

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.

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