Silverleaf nightshade - Weed of National Significance

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Silverleaf nightshade (Solanum elaeagnifolium)

History | Description | Life Cycle | Spread | Impact | Control & Management | Legislation

Silverleaf nightshade is a deep-rooted summer-growing perennial plant from the tomato family Solanaceae.

Silverleaf nightshade may be confused with other Solanaceae species, Quena and Western nightshade. However, there are distinct features which make silverleaf nightshade quite different from other species (Table 1).

The serious reduction of crop and pasture production makes silverleaf nightshade one of the worst weeds in the New South Wales (NSW). It is readily spread by seed and root segments and once established, is very difficult to control.

Silverleaf nightshade contains alkaloids which are toxic to stock although the incidence of poisoning in Australia has been very low as it is generally avoided by most stock.

Silverleaf nightshade is declared a noxious weed in some parts of NSW.

History

Silverleaf nightshade is a native of North America and is the most widespread perennial weed in Arizona. It was first found in Australia in 1901 at Bingara on the north-west slopes of NSW.

Silverleaf nightshade is now found throughout most parts of NSW, south east Queensland, Victoria,  South Australia and Western Australia.

There are slight physiological differences between silverleaf nightshade colonies, which are thought to be due to repeated introductions of the weed into Australia.

Description

The typical silverleaf nightshade is an erect, multi-stemmed perennial plant growing up to 60cm. The leaves are 5-10 cm long, have wavy edges and are silvery-green with a paler undersurface.

Short, brown-yellow spines approximately 5 mm long occur on the stems and petioles (short stems attaching the leaves to main stems). Flowers up to 25 mm in diameter have five purple or white petals with five yellow stamens 7-9 mm in length.

Berries are green striped, round, smooth, commonly 1 cm in diameter which turn yellow-orange when ripe.

Green berries that are only four weeks old and as small as 7 mm in diameter can contain viable seeds. Plants produce up to 60 berries, each containing 10-210 seeds.

Roots of silverleaf nightshade can penetrate to depths of at least two metres. Farmers have even reported roots as deep as five metres. Each plant is usually part of a colony with inter-connecting root systems which helps give silverleaf nightshade its tremendous competitive ability and persistence.

Life Cycle

Silverleaf nightshade regenerates from seed or from rhizomatous root material.

Seedlings emerge at any time from late spring until autumn, depending on rainfall. Emergence occurs more in disturbed soils than on crusted compacted or undisturbed soil. Under favourable conditions up to 80% of fresh seed can germinate. Germination is greatest at depths between 1-3 cm, with optimum soil temperature above 15°C (at 3 cm).

Flowering usually begins in November and can continue until March. Berries are produced from December to March.

Berry formation coincides with carbohydrate translocation to the roots. Plants are dormant in winter when stems die back. The colony then produces new shoots in spring.

Seed longevity

Recent research has shown that silverleaf nightshade seed remains viable in the soil for a much shorter period than originally indicated.

In trials within 36 months 80% of fresh seed had decayed. For managing silverleaf nightshade, three to five years of controlling seedlings and seed set will significantly reduce seed numbers in the soil.

Spread

The spread of silverleaf nightshade is influenced by land use and rainfall patterns. It will grow on most soil types. Silverleaf nightshade will grow from seed and root segments in summer rainfall areas. In southern NSW, with less reliable summer rainfall, vegetative propagation is more common.

Land use, for example cultivation or livestock movement will determine spread. Seed is spread by birds, water, and livestock, particularly sheep. Most viable seed passes through the animal’s digestive system in two weeks, making this a mandatory ‘clean out’ period before stock can be moved to clean areas.

Cultivation breaks the roots into many small segments and can spread them over the paddock. Root segments can travel larger distances in soil on attached to machinery. New plants can regenerate from pieces as small as 1 cm. Research shows that root segments if kept moist can remain viable in soil for up to 15 months.

Impact

A survey of farmers in Victoria, New South Wales and South Australia shows that silverleaf nightshade has spread widely throughout the wheat belt of these states. On average, the total farm cost for silverleaf nightshade was $1730 per year for control and $7786 per year in production losses. Approximately 24% of infestations reported were medium to dense and 76% were rare to scattered. The average infestation size in New South Wales was 218 ha. The most affected land use was cropping 56% followed by grazing 34% (McLaren et al. 2004).

Cropping

Yields of summer broadleaf crops such as cowpeas, soybeans, sunflowers, mungbeans, cotton and horticultural crops can all be significantly reduced by silverleaf nightshade. This is due to strong competition by the weed and the lack of suitable in-crop herbicides. Trials have recorded a yield reduction of 50% in summer crops.

In winter crops, the competition is less obvious. The silverleaf nightshade only directly competes with the crop in late spring. Most of the yield losses are due to the depletion of soil moisture and nutrients in the summer prior to the crop.

One barley trial (1975) found that nine stems per square metre in the fallow reduced the barley yield by 12% in the following winter. Where glyphosate has been used in the fallow to control silverleaf nightshade, wheat yields have increased by 14% in normal years and up to 70% in drier years.

Pastures

Dense infestations of silverleaf nightshade can severely reduce autumn-winter production of annual pastures such as clover and ryegrass. When silverleaf nightshade was controlled, sub-clover production eight weeks after autumn rain increased by 1500 kg/ha dry matter.

Stock

Stock poisoning from alkaloids contained in the silverleaf nightshade has occurred, but this is uncommon. Silverleaf nightshade is only moderately palatable to stock although it is toxic if eaten. Ripe fruit are the most poisonous part of the plant. Symptoms include profuse diarrhoea and profound nervous depression. There can be significant weight loss and eventually death, possibly due to heart failure, after 7 to 14 days of sickness.

Control and management

Silverleaf nightshade colonies are not easily controlled as the extensive interconnecting root systems are difficult to totally control. Silverleaf nightshade has a tremendous capacity to regenerate from root fragments. Continual vigilance is required when managing silverleaf nightshade. Colonies can re-establish even though they may have been controlled for several seasons.

Cropping

Crop rotation allows a variety of herbicides to be used in the control of silverleaf nightshade. Summer cereal crops such as sorghums, maize and millets are tolerant to 2,4-D amine, atrazine and picloram which provide some control of silverleaf nightshade. The use of fluroxypyr in summer crops has also provided some control. Seek advice before using this product as there are some use restrictions in summer cereals.

Winter cropping can be successful if silverleaf nightshade is suppressed during summer. Triazine tolerant canola allows the use of atrazine which can suppress silverleaf nightshade.

Pastures

Good ground cover and competitive perennial pastures can be a key element in controlling silverleaf nightshade. Spring and summer dominant pastures are important as they strongly compete with silverleaf nightshade during its growing season.

Lucerne is deep rooted and dries out the soil profile limiting the colonies’ growth and regeneration. In areas where there is frequent summer rainfall perennial grass pastures provide better competition. Perennial grasses also suppress weed seedling emergence.

When grazing pastures where silverleaf nightshade is present it is important to maintain plant competition to discourage silverleaf nightshade growth. Livestock, particularly sheep, should not be allowed to graze fruiting plants. About 10% of seed that passes through the digestive tract remains viable. Seed can take up to 2 weeks to pass through the gut.

Herbicides

Silverleaf nightshade seedlings need to be controlled immediately after harvest of winter crops or senescence of winter pastures. Silverleaf nightshade seedlings are readily controlled by all registered herbicides. Spot spraying small infestations of seedlings is important to prevent new silverleaf nightshade colonies from establishing. Good herbicide coverage is essential for effective control.

To date there is no herbicide registered that is able to eradicate a silverleaf nightshade colony with a single application. However, colonies can be suppressed and run down with persistent management which includes annual herbicide applications and prevention of berry-set.

When using herbicides on established silverleaf nightshade colonies timing of control is very important to ensure the roots are killed as well as the stems. Spraying colonies is most successful when plants are fresh after rainfall. If the plant is stressed or dormant the herbicides will have little or no effect.

At the end of summer during early berry formation there is usually a movement of nutrients into the root system and herbicides can be carried from the leaves into the roots.

Colonies will require control over several years. Use a number of herbicides in a program and seek advice from your district agronomist on the best program options for your district. Program options could include spraying new shoots in spring and perhaps in early summer with glyphosate to control early growth. This would be followed up by spraying 2,4-D amine at early berry formation to prevent berry-set.

For a list of registered herbicides for controlling silverleaf nightshade in crops and pastures see the Noxious and environmental weed control handbook.

Cultivation

Cultivation is not a useful tool to use against silverleaf nightshade, and is more likely to drag root pieces to clean areas. All parts of the root system are capable of forming shoot buds. Suitable herbicides need to be substituted for cultivation.

Slashing

Slashing is not an option for managing silverleaf nightshade as it recovers readily after slashing even under dry conditions. Silverleaf nightshade can also form berries close to the ground which will not be controlled by slashing.

Biological

After exhaustive studies in North America and Argentina, it was concluded that currently identified natural enemies of silverleaf nightshade would not adapt to the dry summer conditions in southern Australian.

The most damaging agent is the nematode Orrinia phyllobia which forms galls on leaves and stems but it is not suitable as it is not host specific.

The fungi Verticillium dahliai has also been identified as killing isolated scattered plants but is not considered to be of practical significance.

The South African researchers have identified a Chrysomelid beetle (Leptinotarsa texana), which feeds on silverleaf nightshade but it was not released as the agent also attacks eggplant.

Allelopathy

Allelopathy refers to the release of chemical compounds by a plant that inhibit other species.

South Australian experience has shown some eucalypt trees may have allelopathic affects on silverleaf nightshade. The most promising have been Eucalyptus brokwayi (Dundas mahogany), E. dundasii (Dundas blackbutt), E. spathulata (Swamp mallet) and E. salubris (Gimlet gum). These trees gave good control to just outside their drip lines. These species are currently being investigated in NSW.

Quarantine

If silverleaf nightshade is only present in some areas it is vital to quarantine those areas.

Preventing the spread of silverleaf nightshade will save years of problems. Lock stock up for two weeks before moving them onto a clean paddock if they have been grazing on silverleaf nightshade infested paddocks when ripe berries are present.

Clean down machinery for plant fragments when moving from infested to clean areas. Silverleaf nightshade can reshoot from 1 cm of root fragment. Monitor clean paddocks regularly so infestations can be eradicated before they become a problem.

Legislation

Silverleaf nightshade is declared as a Class 3 or 4 noxious weed throughout most of New South Wales under the Noxious Weeds Act 1993.

Class 3 control requirements are ‘the plant must be fully and continuously suppressed and destroyed’.

Class 4 control requirements are ‘the growth of the plant must be managed in a manner that reduces its numbers spread and incidence and continuously inhibits its reproduction’.

Acknowledgements

Authors: Jenene Kidston District Agronomist, Mudgee
Robert Thompson District Agronomist, West Wyalong
Annie Johnson Project Officer, Orange Agricultural Institute, Orange

The authors wish to acknowledge the contribution and assistance of:
G.F. Hennessy, B.R. Milne and J.J. Dellow (Authors of previous edition)
C Bourke, Principle Research Scientist, NSW DPI, Orange Agricultural Institute.
D Lemerle, Principle Research Scientist, NSW DPI, Wagga Wagga Agricultural Institute.
J. Heap, Senior Weeds Research Officer, SARDI.
K. Teague, Animal and Plant Control Board, Melrose, South Australia.
R. Stanton, Post Doctoral Fellow, Wagga Wagga Agricultural Institute.
H. Wu, Weeds Research Officer, Wagga Wagga Agricultural Institute.

References

• Heap, J.W. and Carter, R.J. (1999) The biology of Australian weeds, 35. Solanum elaeagnifolium Cav. Plant Protection Quarterly, Vol 14(1) 2-12.
• Parsons W.T. and Cuthbertson E.G. (2001) ‘Noxious Weeds of Australia.’ CSIRO Publishing: Collingwood VIC. pp 609-612.
• Simmonds, H., Holst, P. and Bourke, C. (2000) The palatability and potential toxicity of Australian weeds to goats. RIRDC. Canberra. p 136.
• McLaren, D.A., Tereso, A.M., Honan, I. and Holtkamp, R. (2004) Distribution, economic impact and attitudes toward silverleaf nightshade (Solanum elaeagnifolium Cav.) in Australia. Proceedings of the 14th Australian Weeds Conference, eds B.M. Sindel and S.B. Johnson (Weed Society of New South Wales, Sydney).