Witchweeds (Striga species)

PROHIBITED MATTER: If you see this plant report it to the NSW Invasive Plants & Animals Enquiry Line 1800 680 244

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Impact

Witchweeds are parasitic herbs that grow on the roots of host plants. They are serious weeds of maize, millet, rice, sugarcane, sorghum and legume crops. Crop losses can be as high as 100%.

Worldwide at least 11 species of witchweed are known to attack crops. The most significant species are Striga hermonthica, Striga asiatica (both on cereals) and Striga gesnerioides (on legumes). All Striga species except for the native Striga parviflora are Prohibited Weeds in New South Wales (NSW).

Parasitic weeds like witchweed are among the most destructive and difficult-to-control weeds in agriculture.

Distribution

Witchweeds are native to tropical Africa, India, the Middle East and China. They infest an estimated two-thirds of all cropping in Africa.

Striga asiatica has the widest distribution and is listed as a weed in 35 countries, including a large infestation in the United States.

In July 2013 Striga asiatica was found on a small number of properties near Mackay in Queensland. Witchweeds are not known to occur in NSW. Specimens of the native witchweed (Striga parviflora) have been collected from woodlands on the north coast and central western slopes of NSW.

Distribution map

Spread

Witchweeds are dependent on a host plant, only germinating when exposed to certain chemicals that host plants give off. A number of witchweed plants can attach to a single host plant.

For Striga asiatica, Striga hermonthica and Striga gesnerioides the most favourable temperatures for germination are 30°– 35°C, and germination does not occur when temperatures are below 20°C. After germinating, plants spend the first four to seven weeks underground obtaining all their nutrients from their host.

After emergence, witchweeds can flower and produce seed rapidly. Each plant is capable of producing at least 50 000 tiny seeds. These may remain viable in the soil for over 10 years.

Seeds are spread short distances by wind, and further by water and soil attached to animals, machinery, tools, footwear and clothing. Contaminated crop seed is the most likely way for witchweeds to be introduced into an area.

Description

The presence of witchweed may be indicated by symptoms in the host plant, which are similar to severe drought stress, nutrient deficiency and vascular disease.

Key identification features

  • Underground stems are round and white. Above-ground stems are four-sided and covered with rough white hairs.
  • Leaves are 6–40 mm long and 4 mm wide, tapering to a pointed tip. They are green and have a rough surface.
  • Flowers (5–8 mm long) occur in spikes (10–15 cm long) atop stems and can be red, pink, white, yellow, orange or purple.
  • Fruits are five-sided capsules about 4 mm long and 2 mm wide, containing around 550 seeds. The seeds are dust-like, 0.2–0.3 mm long, brown and ribbed.

Habitat

Witchweed prefers intensive agriculture where frequent crops, monocultures and fertilisers encourage growth and seed production.

Acknowledgements

2006 edition prepared by Annie Johnson; 2013 edition prepared by Elissa van Oosterhout; Reviewed by Phil Blackmore, Birgitte Verbeek

References

Holm  LG, Plucknett DL, Pancho JV and Herberger JP (1977) The world’s worst weeds: Distribution and Biology. The University Press of Hawaii, Honolulu

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Control

Control is difficult as witchweeds are inaccessible until they emerge, by which time it is usually too late to prevent yield losses. Rotations with trap crops that stimulate witchweed germination can be beneficial. Contact your local council weeds officer for assistance if you suspect you have found witchweed.

Herbicide options

WARNING - ALWAYS READ THE LABEL
Users of agricultural or veterinary chemical products must always read the label and any permit, before using the product, and strictly comply with the directions on the label and the conditions of any permit. Users are not absolved from compliance with the directions on the label or the conditions of the permit by reason of any statement made or not made in this information. To view permits or product labels go to the Australian Pesticides and Veterinary Medicines Authority website www.apvma.gov.au

See Using herbicides for more information.


PERMIT 9907 Expires 31/03/2020
Fluroxypyr 333 g/L (Starane™ Advanced)
Rate: 300 to 600 ml in 100 L of water
Comments: Spot spray application
Withholding period: 7 days.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate


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Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area Duty
All of NSW General Biosecurity Duty
All plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.
All of NSW Prohibited Matter
A person who deals with prohibited matter or a carrier of prohibited matter is guilty of an offence. A person who becomes aware of or suspects the presence of prohibited matter must immediately notify the Department of Primary Industries
All species in the Striga genus are Prohibited Matter in NSW, except the native Striga parviflora

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For technical advice and assistance with identification please contact your local council weeds officer.
For further information call the NSW Invasive Plants and Animals Enquiry Line on 1800 680 244 or send an email to weeds@dpi.nsw.gov.au

Reviewed 2017