Description

Dodder is a serious parasite of crops and pastures. Dodder is the common name for species belonging to the genus Cuscuta. There are about 170 species of Cuscuta, 14 of which are found in Australia.

Golden dodder (Cuscuta campestris) is easily seen because its thin thread-like stems are bright yellow and stand out clearly on the green background of the crop or pasture. A related species, Chilean dodder (C. suaveolens), is low-growing and pale yellow/green, making it much less visible.

Both species are annual, leafless plants which parasitise host plants by suckers, called haustoria. The dodder stems twine around the host, branching to form a tangled mass which can spread from the initial host to adjacent plants. The haustoria penetrate the tissues of the host plant and remove nutrients, thereby reducing crop or pasture yields, preventing host development or even killing it. Contaminated seed or other produce is unmarketable.

Golden dodder is a threat to lucerne, vegetables, many broadleaf crops, pastures and seed crops.

Lifecycle

Seeds germinate in soil from September through to March/April. The emerging seedling must make contact with a suitable host plant within two to six days, otherwise it dies. However, in wet conditions seedlings can survive for several weeks without a host.

Having attached to a host, the thread-like dodder stems spread over and around it in a tangled mass, extracting nutrients as it grows.

Despite having no true leaves or roots, golden dodder grows rapidly, flowering and seeding throughout the warmer months (September-May).

Golden dodder produces clusters of small white flowers which mature into capsules containing up to four small seeds. The seeds are slightly pear-shaped and similar in size to clover and lucerne seed.

The seeds can be spread in contaminated fodder and pasture seed. They also spread readily in water by natural streams or by irrigation. Animals can also spread the seed in faeces and in mud on their hooves.

Occurence

Golden dodder is the most common introduced species in New South Wales.

It is widespread along the State's rivers, creeks, floodplains and irrigation areas, because water is the principal method of spreading and the availability of suitable host plants and livestock are concentrated along waterways.

While the most important host crop in Australia is lucerne, dodder has been recorded on a wide range of different field crops, pasture legumes, vegetables and horticultural plants.

Many weed species and tree seedlings are also suitable hosts, allowing golden dodder build-up in waste areas where it is not controlled.

Control - a long-term approach

Golden dodder is a difficult weed to eradicate. Its seed can survive for long periods, it grows rapidly and can commence seeding after only a few weeks' growth. It will grow on a wide range of host plants including crops, vegetables, weeds and some tree species.

A long-term approach is required to control this weed. Prevent it entering your property and be vigilant in spotting and destroying new outbreaks before they seed.

Prevention is best

Stopping the entry of golden dodder onto your farm is the best control measure. Since it can spread as seed, or as plant fragments care is needed not to scatter either.

Golden dodder is not readily eaten by livestock but to reduce the spread, contaminated hay and feeds should not be used and grazing infested pastures should be avoided.

Sow clean seed

Pasture seed infested with golden dodder is an important source of new infestations. Because of their similar size, dodder seed is very difficult to remove from lucerne and small clover seeds. It is essential, therefore, to use certified seed.

Buy clean fodder

Contaminated hay and grain can introduce golden dodder to a clean property. Hay containing golden dodder should be destroyed as mature seed is often present. If fed to stock, golden dodder may cause intestinal disorders in horses and cattle, and in some literature is suspected of poisoning livestock.

Restrict seed movement

Machinery used for fodder conservation in infested paddocks should be cleaned before moving to other paddocks.

Stock grazing on areas known to be infested with golden dodder should be kept in 'quarantine' for at least 2 days before they are moved to dodder-free pastures. Stock can transport seed and even pieces of plant. Golden dodder seed can remain viable after ingestion and it is known to grow in the dung of cattle, sheep, goats, kangaroos and pigs. Birds can also excrete viable seed.

Wherever infestations are found they should be marked and the area quarantined from activities such as hay-cutting and grazing. Care should also be taken to avoid spreading seed in soil, on machinery or boots.

Golden dodder is commonly found along streams and in flooded areas, as water is a common means of spread. However, mature golden dodder seed does not float and cannot be transported over long distances, but the seed is often carried on trash or in seedcases that can float in running water.

Golden dodder seed can also be spread by irrigation. The very small seed can easily be picked up in irrigation suction lines. Filters can be used but the main obstacle to simple filtration is where sand is carried in the water. Green weed could also present filtration problems. Self-cleaning and flushing filters are available.

Methods of control

Early detection is necessary if golden dodder is to be controlled before seeding commences. Flowering can begin within 2 weeks of germination. Lucerne paddocks should be inspected thoroughly before first cut, to detect new golden dodder seedlings. Similarly, pasture paddocks need to be inspected after rainfall in late spring. If golden dodder is found, control strategies need to be implemented immediately.

Mapping existing infestations at the end of summer will also help in checking reinfestation the following spring.

All workers on the farm should be made aware of golden dodder and be able to identify it.

Several options are available for golden dodder control, depending on the size of the infestation, the crop or pasture infested, and the crop or pasture rotation available.

Whichever option is chosen, it is essential that treatment should begin as soon as possible after an infestation is noticed. Otherwise what may begin as a few scattered plants will quickly build up, resulting in a major infestation. This, in turn, severely reduces productivity and requires much time and cost to control the outbreak.

The control of weeds that are the preferred hosts for golden dodder will reduce its chance of spreading and prevent them becoming sources of infestation. These include Noogoora and Bathurst burrs.

Since golden dodder can be spread by seed and by its extending tendrils, it is important to always treat outside the obvious areas of infestation. Where spot spraying or burning small infestations, it is easy to overlook small threads starting to invade further into the crop. These threads, even as small as 5 cm, can re-attach to the host crop and continued infestation will result.

A buffer zone of at least one metre diameter outside the initial infestation should be treated or burnt to prevent this occurring.

Once treated, infestations should remain marked so the effectiveness of the control can be checked at a later date.

Cutting or burning

Golden dodder can be controlled by cutting the host plant as close as possible to ground level and burning it.

However, the area should be inspected again after one week, as golden dodder may re-shoot from the crowns of lucerne and weeds such as wireweed.

Do not cut or slash golden dodder and leave the cut material in susceptible crops. Golden dodder will re-attach itself to new hosts. Slashing promotes the spread of dodder.

All host and golden dodder plant material must be burnt, preferably on the infested site.

Burning can be carried out after cutting and drying. Use flame throwers or other combustible material such as old hay or distillate. Burning is more effective following application of a desiccant herbicide, or diesel.

The hotter the burn the better the kill of dodder, but the greater the risk of killing the host plant, eg lucerne.

Chemical control

Although no truly selective herbicides are available for use, contact herbicides can be used to rapidly desiccate the standing crop and destroy the golden dodder.

It is likely that golden dodder seed may already have matured, so the sprayed area should be burnt after 2-3 days to ensure its destruction.

Treatment with contact herbicides alone will not destroy mature seed. A lucerne stand which has been treated with a contact herbicide, or burnt, can regenerate from undamaged crowns for continued production without the need for re-sowing.

Contact herbicides act by quickly killing green plant material. They are rapidly absorbed and are not affected by rain after spraying. On contact with the soil they are inactivated and therefore do not harm crop roots or seeds below the soil surface. There are no harmful soil residues and stock need only be withheld for seven days. For best results thorough coverage and good penetration of foliage is necessary.

Where golden dodder infestations occur in non-crop areas, golden dodder can be eradicated by control with non-selective herbicides. Damage to other plants will result where these herbicides are used.

Golden dodder can be eradicated by control of the weeds which it is using as a host. Consult the handbook Herbicide control of noxious weeds for chemicals registered to control Dodder.

Crop rotation

Lucerne or clover stands which are severely infested should be cultivated and replaced with less susceptible crops or pastures.

Cereal crops such as wheat, barley, oats, triticale and cereal rye are poor hosts to golden dodder. Summer grain crops such as maize and sorghum are resistant to golden dodder.

While golden dodder cannot complete its life cycle on cereal crops, it may do so on broadleaf weeds which invade these crops. The advantage of a crop rotation for golden dodder control is lost if host weeds occur in the crop. In-crop weed control then becomes an essential part of crop management.

If winter cereals are to be grown in an infested paddock a clean fallow must be maintained over the summer months, when golden dodder is active. This can be achieved with herbicides or cultivation.

Pasture rotations will need to be based on perennial grass species where practical. Subterranean clover can be used as the legume component, but any summer-growing broadleaf weed, or legume, will need to be controlled to prevent golden dodder reinvading.

Where crops or pastures are used in rotations to control golden dodder, lucerne or susceptible legumes should not be sown for at least 5 years.

Deep ploughing can help reduce the seed burden by burying golden dodder seed. Most golden dodder seed will not germinate from a depth of greater than 7.5 cm.

When re-sowing lucerne or susceptible clovers, late summer to autumn sowings are less likely to be infected by golden dodder during the establishment period.

Biological control

No biological control agent is available in Australia for golden dodder. Research has been conducted for many years overseas, but there is no likelihood of any release in the immediate future.

Follow-up control

Continued vigilance is needed even after control measures have been implemented. Lucerne growers should be wary of reinfestation.

Under ideal conditions, the seed of dodder can survive in the soil for up to 20 years and some plants could still reinfest paddocks several years after a successful control program. It is essential that a follow-up control program be implemented, otherwise initial work may have been in vain.

Dodder is proclaimed noxious throughout New South Wales Under the Noxious Weeds Act 1993, it must be controlled. Local councils are responsible for enforcing this act. The landholder is responsible for controlling Dodder on private land, and failure to do so will result in a fine.

The authors wish to acknowledge the assistance of Hugh Milvain and Peter Gorham in the preparation of this publication.