Witchweeds (Striga species)

WEED ALERT: STATE PROHIBITED WEED
If you see this plant contact your council weeds officer, the NSW Invasive Plants & Animals Enquiry Line 1800 680 244 or email weeds@dpi.nsw.gov.au

Profile

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 Maps

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 (Staraneā„¢ 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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Legal requirements

The content provided here is for information purposes only and is taken from the Noxious Weeds (Weed Control) Order 2014 published in the NSW Government Gazette, detailing weeds declared noxious in New South Wales, Australia, under the Noxious Weeds Act 1993. The Order lists the weed names, the control class and the control requirements for each species declared in a Local Control Authority area.

All species in the Striga genus are declared except the native Striga parivflora

Area Class Legal requirements
All of NSW 1 State Prohibited Weed
The plant must be eradicated from the land and that land must be kept free of the plant

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Witchweeds have delicate flowers (5-8 mm) at the tops of stems.
Witchweeds have delicate flowers (5-8 mm) at the tops of stems. (Photo: Courtesy USDA APHISS PPQ Archive Bugwood.org)

Witchweeds are parasitic plants that cause severe damage in crops.
Witchweeds are parasitic plants that cause severe damage in crops. (Photo: Courtesy USDA APHISS PPQ Archive Bugwood.org)

Flower of Stiga hermonthica.
Flower of Stiga hermonthica. (Photo: Michael Jones, Newman College, Birmingham)

Witchweed flowering through a crop.
Witchweed flowering through a crop. (Photo: Courtesy USDA APHISS PPQ Archive, Bugwood.org)

Witchweeds are parasites that grow on the roots of host plants.
Witchweeds are parasites that grow on the roots of host plants. (Photo: Courtesy USDA APHISS PPQ Archive Bugwood.org)

Flowers occur on spikes at the tops of stems.
Flowers occur on spikes at the tops of stems. (Photo: Courtesy Florida Division of Plant Industry Archives Bugwood.org)

Flower colour can be red, pink, white, yellow, orange or purple.
Flower colour can be red, pink, white, yellow, orange or purple. (Photo: Courtesy USDA APHISS PPQ Archive, Bugwood.org)