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Spotlight on sunflower broomrape

December 7, 2018

Orobanche cumana (sunflower broomrape) is a parasitic plant and substantial threat to sunflower production in countries around the Black Sea and across Europe.

History of broomrapes

Orobanche was first identified as a problem soon after sunflowers started to be commercially grown in mid-19th century Russia, with the first recorded infection appearing in the central Russian region of Voronezh in 1866.

Orobanche had probably been affecting sunflowers before then but it would have taken scientists some time to identify it as a problem as the plant spends most of its time underground, only emerging late from the soil to flower and set seed.

Distribution of broomrapes

The spread of Orobanche followed the expansion of sunflower production from Russia, across Ukraine, Moldova, Romania, Bulgaria, Hungary and it is now present in all countries where sunflowers are grown around the Black Sea and Southern Europe.

It is also now affecting crops further afield, including Spain where it was first reported in 1958 and more recently France in 2009 while another species of broomrape, listed as Orobanche ramosa, turned up in South Australia in 1992 and has been the subject of an eradication program.

All indications are that the spread of Orobanche continues across the sunflower growing regions aided by its highly successfully ability to spread seeds and the rapid evolution of more aggressive biotypes.

Broomrape species are thought to infest 16 million hectares of crop land worldwide and affect around 50% of the world’s sunflower crops while in some region’s broomrape has become such a problem that growers have abandoned the production of susceptible crops altogether.

Biology of broomrapes

Broomrapes are annual plants that establish themselves from seed each year.  They lack chlorophyll and therefore cannot photosynthesis; instead, they derive their supply of carbon, nitrogen and inorganic solutes by parasitizing the roots of host plants.

Key biological characteristics

  • Broomrape plants spend most of their time underground.
  • Stems emerge start 35 days after planting depending on soil type, soil moisture, air and soil temperature and the host plant.
  • Flowering and seed production take place 18-20 days after emergence.
  • Seeds are very small, 200-300 µm, and like ground pepper in appearance.
  • The number of seeds per flower is 2.500 –500. Individual Broomrape plants can produce more than 87.000 seeds
  • Seeds are easily spread by wind, water, machinery, livestock and contaminated crops.
  • Seeds can become electrostatically charged and cling to metal and plastic.
  • Seeds remain viable for decades once buried in soil.

Broomrape seed germination is a two-step process requiring conditioning of seed in moist warm soil (20 to 25°C) and then the presence of a suitable host which exudes chemical compounds in to the rhizosphere surrounding its roots.

When conditions are right for germination, the broomrape seed produces a root-like germ tube that attaches itself to the host root, the germ tube is only a few millimetres long and if it does not attach to a host root in time, it quickly dies.

A successfully attached germ tube uses a combination of mechanical action and enzymes to penetrate the hosts root epidermis and connect with the vascular system; once attached the broomrape draws nutrients and water from its host and is parasitic.

Economic importance of broomrapes

Orobanche is now considered to be the most important biotic constraint to sunflower yield right across Southern Europe and the Black Sea.

Sunflower plants infected with orobanche are stunted with smaller heads and yield losses are reported to be anywhere from 20% to 100%.

Phytosanitary implications of broomrapes

Orobanche cumana, like many other broomrape species is listed and restricted under the phytosanitary regulations of most countries.

The seed is easily transported in contaminated crop seed as visual dentification of the small seeds is extremely difficult, however molecular methods have recently been developed that allow the number of parasite seeds per kilogram of crop seeds to be determined.

Management strategies for broomrapes

The success of broomrape lies in its ability to rapidly form a substantial seed bank in soil.  Any management strategy must aim at reducing the seed bank while minimising the production and dispersal of new seeds.

Trap crops

Researchers have identified possible susceptible plant species such as rye, corn, sorghum and switchgrass, that could be used to stimulate germination of Orobanche seeds and help reduce the soil seed bank, but this has not yet been reported in a field situation.

Mechanical Control

Carefully removing shoots by hand may reduce the seed bank if completed without shedding any seed but that would be difficult to achieve over large-scale commercial crops plus the benefit to the existing crop would be limited as most of the damage would have already been done by then.

Biological Control

Researchers have identified possible biological control organisms including Fusarium and Aspergillus fungi and the fly Phyomyza orobanchia whose larvae prevent seed production in the seed capsule, however, these are not commercially available.

Chemical Control

Chemical control strategies should aim to eliminate host weeds to reduce Orobanche seed in the seed bank and control Orobanche within susceptible sunflower crops.

Eliminating host weeds requires good all-round agronomy throughout the crop rotation to reduce the opportunities for Orobanche to germination and produce yet more seeds

This can be done with selective weed control in crops or total vegetation control by using none selective herbicides, such as glyphosate, between crops or a combination of both.

Controlling Orobanche in a susceptible crop becomes more of a challenge as the plant spends most of its life underground and has no photosynthetic system making it difficult to control by conventional herbicide strategies.

Chlorsulfuron, imazapic, imazapyr, imazethapyr, oxyfluorfen, sulphylurea and trifluralin have all been shown to offer varying levels of control but were also critical of timing.

Any herbicide control should be used in conjunction with local pesticide legislation.

Host Resistance

Effective control of Orobanche in sunflower is largely based on using resistant varieties which was first recognised in Russia in 1912 and research has been going on ever since.

As new resistant sunflower varieties are developed, more virulent races of Orobanche appear every 20 years meaning research into host plant resistance needs to continue.

Clearfield® Varieties

Clearfield® and Clearfield® Plus Production System use conventional plant breeding techniques to develop varieties of sunflowers that are tolerant to imidazolinone herbicides, while conventional sunflower varieties are susceptible to imidazolinone herbicides.

This allows for effective season-long weed control to a broad spectrum of broadleaf and grass weeds including broomrape.

Integrated Pest Management

Effective control of Orobanche relies on an IPM approach incorporating some or all the above management strategies across seasons and across crops.

Latest situation of broomrape

Bulgaria

The appearance and distribution of new Orobanche biotypes has increased in line with sunflower production, although the development and use of highly resistant varieties has helped to contain the problem.

Hungary

Research suggests the risk from sunflower broomrape might increase because of dry conditions and the appearance of more virulent Orobanche biotypes, especially in the South-Central to North-West regions.

Republic of Moldova

The occurrence of broomrape in the Republic of Moldova was first mentioned in 1937 and today all races of Orobanche are present on the territory and the affected areas have increased.

Romania

Orobanche has become more of a problem in Romania with 60% of the sunflower cultivated area now infested with broomrape, the most virulent biotypes are in the South-Eastern Dobrogea region.

Russian Federation

The most virulent Orobanche biotypes are now found in Rostov, Voronezh, Volgograd, Saratov, Orenburg, Stavropol and Krasnodar regions.  In northern Krasnodar bordering with Rostov broomrape has begun to spread there after a long absence.

Ukraine

Ukraine is one of the world’s largest producers of sunflower and an estimated 70% of the crop area is now infested with broomrape, driven by an increase in the cultivation of sunflower and short rotations.  Sunflower broomrape is the most important problem in the sunflower crop in the South and South-East part of Ukraine.

The Future

Researchers will continue to look at novel techniques for controlling and eradicating Orobanche with some interesting developments using trap crops, biological control and seed treatments offering potential.

Plant breeders need to continue the search for more resilient resistances genes to stay ahead of the parasites ability to evolve and overcome host plant resistance with increasingly more aggressive biotypes.

Combining herbicide and genetic resistance could supply more durable resistance for broomrape control in the future.

Clearfield technology is offered as a control option.

Contact Nuseed for more information about their Orobanche resistant products.

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