Weeds in Australia

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Orobanche spp.

Description
 Broomrapes (Orobanche spp.) are small, erect, branched or unbranched, parasitic herbs, completely lacking in chlorophyll and dependent on other plants for their food supply; they gain their food by attaching to the root system of the host plant. There are three species in Australia, two introduced, Branched Broomrape Orobanche ramosa and Common Broomrape O. minor, and one native species (O. cernua var. australiana) which is not considered weedy and is becoming increasingly rare. Under Victorian legislation, only O. ramosa is listed.

Because of the lack of chlorophyll they generally have a brownish appearance. Gland-tipped hairs may be present on both the stems and in the inflorescence. They have small scale-like leaves arranged alternately along the stems and similar bracts below the flowers. Bisexual flowers are formed towards the apex of the stem, sometimes occupying much of the stem length. The calyx may be joined into a tube at the base and have five equal parts (Branched Broomrape) or it may have four or five unequal parts which do not form a tube at the base (common broomrape and O. cernua var. australiana). The flower is tubular at the base, in Branched Broomrape widening into a throat, but in common broomrape and O. cernua var. australiana, narrowing into the throat. The flower is 2-lipped, more noticeably so in Branched Broomrape than in the other two species. Flower colour is pale blue in Branched Broomrape and common broomrape, but the latter species has darker venation, while flowers are dark blue-purplish in O. cernua var. australiana. The fruit is a capsule containing large number of microscopic seeds (Barker 1999; Faithfull & McLaren 2004).

Because of their parasitic nature many Orobanche species are not desirable. Branched Broomrape is particularly unwanted because of its ability to form attachments to the roots of a very large number of economic plants and significantly reduce yields. The presence of seed in harvested crops may be sufficient to have the crop rejected by an importing country. It is currently only known from South Australia (W.R.Barker 2007, pers. comm.). Common Broomrape is considered to be much less of a problem because it only attaches to plants of much less economic significance. Its spread around Australia has been insidious and it is now widespread in the southern states (AVH 2007).

For further information and assistance with identification of broomrape contact the herbarium in your state or territory.

Distribution:Apart from an early record from the Glenelg sandhills, Branched Broomrape is presently confined to a small quarantined area in the Murray mallee region of South Australia (Faithfull & McLaren 2004; Holding 2004).

Common Broomrape has a much wider distribution and is found in all states except Northern Territory. It is particularly common in south-west Western Australia and in south-eastern Australia in crop-growing areas from south-east Queensland to the Mt Lofty Ranges in South Australia. It has been in Australia for a much longer period of time since with collections from the mid to late 1800s (W.R.Barker 2007, pers. comm.).

The Native Broomrape species is found in coastal Western Australia and along the Murray River in Victoria, but its main distribution is in inland South Australia towards Lake Eyre and in north-western New South Wales (AVH 2007).

Habit:Herb
Key points:
  • In Australia, there are currently two introduced species and one relatively rare native species of orobanche.
  • Branched Broomrape (Orobanche ramosa) is a parasitic plant whose hosts include many crop plants.
  • Because of its microscopic seed it is often found as a seed contaminant in those crops where it grows.
  • Contamination with broomrape seed means rejection of crops by importing countries.
  • An understanding of its biology and hence the best form of control is still to be achieved.
How it spreads:

Broomrape seed is microscopic and easily carried by wind, by water or by animals or machinery. Estimates of half a million seeds produced per plant have been cited (Lloyd et al. 2007). Possible vectors cited for distribution of the seed include farm machinery, livestock, soil and harvested crops or fodder (Secomb 2006).

 

Where it grows:There does not appear to be any particular preference for habitat for any of the broomrape species. Branched Broomrape occurs in alkaline sandy soils in the Murray mallee region, while common broomrape and the native species are frequently found in sand hills which might suggest that sandy soils are the best for growth of the haustorium which forms the attachment between the broomrape and the host species. However, one specimen of the native broomrape has been recorded from heavy black clay in Victoria (Barker 1999: W.R.Barker 2007, pers. comm.).

Distribution will presumably be governed by the presence of suitable hosts. Since the number of potential host plants for Branched Broomrape is large, it is a threat to any area where such plants occur. Host plants documented elsewhere include Celery, Chickpea, Tomatoe, Lettuce, Coriander, Onions, Potatoe, Sunflower, Tobacco, Canola, White Mustard, Cabbage, Vetch, Carrot, Faba Bean, Cannabis and Clover, as well as a number of weeds and even some native daisies (Faithfull & McLaren 2004; Department of Water, Land and Biodiversity Conservation. updated 2007).

Common Broomrape is a weed of crops and gardens in temperate regions and is occasionally also found in subtropical environments (Navie 2004).

Flower colour:Blue, White, Purple
Distribution map: http://www.flora.sa.gov.au/mapper.html 

In South Australia http://www.environment.sa.gov.au/soe2003/sup_report/biodiversity/introduced.pdf 

Impacts:Broomrape was included in the list of 71 species that were nominated by state and territory governments for assessment as Weeds of National Significance (WONS). Following an assessment process, Broomrape Bush was not included as one of the 20 WONS. However, it remains a weed of potential national signficance.

Branched Broomrape is such a devastating parasitic weed that it has been known to reduce crop yields by up to 90%. It parasitises a wide range of crops, including vegetables such as Carrots, Tomatoe, Potatoe and beans and also legume crops. A farming area of South Australia has been quarantined for at least 12 years in an attempt to prevent its spread in Australia (cost for one year of quarantine alone is $4.5 million) (Warren 2006). The seed are so microscopic that it is easily spread by a variety of means and also easily contaminates seeds of harvested crops (Holding 2004; W.R.Barker 2007, pers. comm.).

Common Broomrape is a less aggressive species, which attaches to such plants as Gazania and Clover (W.R..Barker 2007, pers. comm.).

Origin:

Broomrapes are mostly native to Europe and Asia and there are about 140 species worldwide. Some species are more undesirable than others, but Branched Broomrape, Egyptian Broomrape (O. aegyptica), Nodding Broomrape (O. cernua var. cernua), Crenate Broomrape (O. crenata) and Sunflower Broomrape Orobanche cernua var. cumana [as O. cumana] appear to be the worst of them (Faithfull & McLaren 2004). Branched Broomrape is banned in many countries of the world (Scher undated).

 

History:It is not known how the introduced species of broomrape first arrived in Australia.

The timing of the introduction of Branched Broomrape into South Australia is disputable. There is a single 1911 collection of the species from near Glenelg in the State Herbarium of South Australia. There had been no further suggestions of its presence until a collection from the Murray mallee area was brought into the State Herbarium of South Australia for identification in 1992 (Holding 2004; W.R.Barker 2007, pers. comm.).

Common Broomrape has been in Australia for a much longer period of time since there are collections from the mid to late 1800s (W.R.Barker 2007, pers. comm.).

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This database is designed to provide information, including biological and ecological, on invasive plant species that are on a national weed list, or are legislated against in a state or territory. While reasonable efforts have been made to ensure the accuracy of the information, no guarantee is given, nor responsibility taken, by the Commonwealth for its accuracy, currency or completeness. The Commonwealth does not accept any responsibility for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the information contained in this database. The information contained in this database does not necessarily represent the views of the Commonwealth. This database is not intended to be a complete source of information on the matters it deals with. For further information on the images contained in the database please contact the copyright owner. All images in the weed identification tool are managed by the Australian Plant Image Index (APII). Various copyright conditions apply for these images. For further information on the copyright conditions of images contained in the database please contact the APII at: photo@anbg.gov.au.