The parasitic plant has many mutations and is spreading aggressively around the world. A range of measures should be taken to control it.
One of the most dangerous enemies of sunflower in the world is recognized as the sunflower broomrape. Every year this parasitic plant spreads more and more in Ukraine, moving from the south and east to other regions, affecting previously resistant hybrids and causing significant crop losses. The situation is further complicated by the fact that the fight against broomrape is an extremely complex matter. Therefore, for scientific and practical substantiation of the real prospects for defeating the parasite, we turned to Dmitry Shatsman, Director of the Evrosem company. “Many seed companies speculate on this topic, claiming that they have sunflower seeds with resistance to broomrape races 9 and 10,” he noted. “In fact, this is manipulation.” In order to identify myths and use effective methods of fighting broomrape, the expert recommends delving into the topic.
Record-breaking parasite
Broomrape belongs to the Orobanchaceae family, which has about 1,600 species. Today, broomrape affects 16 million hectares of global sunflower crops, and about 50% of global crops suffer from its influence in one form or another.
Sunflower broomrape is an obligate parasite that exists only in the presence of a host plant. This herbaceous plant lacks chlorophyll and is not capable of photosynthesis, and it receives all its nutrients from the sunflower, parasitizing on the plant’s roots. Broomrape has a simple or branched stem of a yellowish or reddish color, has an average height of up to 40 cm, but sometimes grows up to 110 cm, which was recorded by Evrosem specialists.
Each plant produces from 30 to 120 flowers and is capable of forming up to 300 thousand seeds, which is included in the Guinness Book of Records as the smallest. With more than 2 flower stalks, the yield of sunflower begins to decrease significantly.
In the CIS countries there are up to 20 species of relatives of sunflower broomrape, in Ukraine there are five: hemp or branched broomrape, sunflower broomrape, melon broomrape (affects about 70 species of plants, including potatoes, tomatoes, tobacco, melons), cabbage broomrape and alfalfa broomrape.
Why sunflower?
Once in the soil, sunflower seeds begin to germinate and secrete root exudates to attract arbuscular fungi for symbiosis (this is how the sunflower obtains hard-to-reach nutrients from the soil).
During the process of evolution, broomrape, like arbuscular fungi, received a gene that is able to recognize the “signal” of sunflower to germinate in the direction of the root of the host plant.
Sunflowers secrete the greatest amount of exudates in a state of “starvation” caused by drought, deficiency of nutrients in the soil, in particular, due to disrupted crop rotation. Such conditions are widely observed in the south and east of the country, which means that in those regions sunflower plants secrete more root exudates than usual.
Daring penetration
Having received a “signal” from the sunflower, broomrape begins to move its root tube in the direction of the high concentration of plant exudates, penetrates the root and attaches itself to it. The main tool for paratizing broomrape is a modified root system (haustorium). The main task of this organ is to grow together with the sunflower root, penetrate the arteries and establish intervascular connections with the host plant.
Penetration into the conducting tissues acts as a trigger for the beginning of the growth of the embryonic bud and the accumulation of nutrients in it for the formation of the shoot. In sunflower plants affected by broomrape, the water and nutritional regime of the roots and above-ground part is disrupted. They slow down growth and development, which in turn leads to significant losses in the sunflower crop.
The final stage of development of sunflower broomrape is the emergence of the shoot on the surface and the formation of seeds. “Since the main organ of broomrape is the haustorium, then inter-row cultivation in the fields affected by the parasite does not lead to the destruction of shoots,” notes the director of Evrosem.
How fast?
Broomrape takes over a field in four crop rotation cycles. In the first, individual plants are affected. In the second, there is an expansion from the primary lesion by 10-15 meters.
In the third cycle, the secondary damage zone increases to 200-300 meters, the farm suffers crop losses – broomrape is noticed. The fourth cycle is a complete damage of the field, in which economic losses can reach 60-80% of the planned harvest.
That is, if sunflower is a monoculture, then in four years the field will be infected with a parasite. If 3-4-field crop rotation is observed, this will happen over a much longer period and only if the owner does nothing.
“Accomplices” – ignoramuses
The main friend of the sunflower broomrape, oddly enough, is man. After ripening, the seeds fall off and get no further than a meter from the mother plant. But due to their electrostatic properties, the seeds stick well to the reapers and wheels of tillage machinery and grain trucks, plant residues and sunflower seeds. Harvesting equipment has played a particularly negative role in the spread of broomrape. This is confirmed by the fact that the edges of the fields are affected more than the middle.
“Ploughed seeds can retain their germination capacity in the soil for up to 20 years. Therefore, the recommendation to use deep ploughing to control broomrape is not effective,” Dmitry emphasizes. “In addition, seeds can germinate from any depth, the plant can form flowers and seeds under the soil surface.” The optimal conditions for the germination of broomrape seeds are a soil temperature of 16-25 °C and humidity of 70-75%. At temperatures below 10 °C and above 35 °C, the seeds do not germinate, and at temperatures above 50 °C, they die.
Another unprofessional advice concerns the use of corn as a trap for broomrape seeds. There is a theory that corn with its root secretions provokes the germination of broomrape seeds, but its sprouts die because the “queen of the fields” does not allow them to penetrate its root. Research conducted over several years has shown that the effectiveness of corn as a trap crop for broomrape germination is 2-10%.
In order not to bring broomrape from a neighbor when renting equipment, it is necessary to wash the wheels and the lower parts of the body. Harvesting should begin with clean fields, and affected ones – last.
It is recommended to carefully examine the edges of fields where fallen grain from grain trucks sprouts. Such seeds lose their genetic value and are very easily parasitized by broomrape.
“You can practice sowing second-generation sunflower varieties or hybrids by continuous sowing in order to provoke germination and subsequent destruction of the crop. But not earlier than 40 days after the broomrape seeds have ripened,” the expert advises.
In order not to spread broomrape with seed material, you need to buy only certified seeds, because they undergo field inspection and laboratory analysis. It is recommended to use seeds of sunflower hybrids of the ClearField (Basf) system.
The identification is correct
Broomrape first appeared on sunflowers at the end of the 19th century, when industrial production of the crop began. The parasite moved to the oil crop from wormwood. Since then, broomrape has mutated every 15-20 years and formed a new race, thus “responding” to the emergence of new selected varieties and hybrids.
In the 2000s, when sunflower fields began to expand and crop rotation was increasingly violated, broomrape became the number one threat to crops. “The main problem in the fight against the parasite is that the mutation of broomrape races occurs locally. That is, the parasite could overcome the resistance of seven hybrid races in one region, but could not overcome other hybrids in another region,” explains Dmitry Shatsman. In 2019, specialists from the broomrape research department at Evrosem examined 16 locations affected by the parasite and found that in each region, in each field, broomrape has morphological differences. This means that they cannot be called one, second or third broomrape race, because they are not genetically identical.
In other words, the selection of resistant hybrids and donors of resistance for these hybrids must be carried out in the area where their production is planned.
A task for breeders
Sunflower has two types of resistance to broomrape. Qualitative (vertical) resistance is usually controlled by single genes. It is not long-term, since under the influence of selection new types of broomrape appear that affect resistant hybrids.
A complex mechanism is quantitative (horizontal) resistance – it is caused by several genes and gives a long-term effect. Hybrids with such a mechanism are more difficult to create, since it is necessary to use several resistance genes from several donors.
It is interesting that hybrids are endowed with different resistance mechanisms. Thus, hybrids resistant to race B have characteristic changes in the chemical composition of exudates for the germination of broomrape seeds; and also a hypersensitivity reaction, when the sunflower root cells die at the point of contact with the broomrape sprout cells. Hybrids resistant to race C are characterized by strengthening the cell wall when the broomrape sprout penetrates by cross-linking proteins.
The main donors of genes for resistance to new virulent races of broomrape (G and H) are wild perennial forms of sunflower, such as Jerusalem artichoke. In new hybrids, resistance to broomrape manifests itself differently: the outer cells of the sunflower root have certain physiological and biological characteristics, due to which haustorial cells immediately die upon penetrating the root.
The Evrosem portfolio includes two sunflower hybrids that are resistant to 7+ broomrape races and have horizontal resistance: the ultra-early TOR 7+ and the drought-resistant AVALON. In addition, the manufacturer has developed the Sumo technology for a hybrid that is resistant to six broomrape races.
Throw in all your strength
The fight against broomrape is only gaining momentum: scientists and producers from different countries of the world are uniting to consolidate their efforts to solve the problem. It has been studied that the parasite has a local mutation; therefore, the main problems are the genetic identification of new broomrape races and the search for resistance donors.
Evrosem is the only company with Ukrainian roots and funding that has joined the international consortium to study and combat broomrape. Last year, the company tested 300 broomrape resistance donors at its sites and collected more than 100 samples of O. cumana from different regions of the country to conduct genetic research and greenhouse tests. This year, the company doubled the number of test sites to 10 and began field research on various hybrids.
Over the next five years, Evrosem plans to actively work towards finding not only resistant hybrids and resistance donors, but also finding stimulants for the germination of broomrape, synthetic or organic herbicides that can destroy broomrape.