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The most numerous group of obligate underground parasites is broomrape. The genus broomrape - O robanche, in comparison with other genera of the family, is distinguished by an exceptional diversity of species composition (up to 120 species are known). This is explained by a wide distribution area and a large range in the choice of host plants. Representatives of the broomrape genus parasitize wild, cultivated and weed plants.
Within our country there are more than 40 species of broomrape, including five parasites of cultivated plants. The most harmful are the following species infecting industrial, fodder, ornamental, vegetable, melon crops : sunflower broomrape - O. sitapa, branched broomrape, or hemp, - O. ramosa, Egyptian broomrape, or melon, - O. aegyptiaca, mud broomrape - O mutellii and alfalfa broomrape - O. lutea.
In the process of evolution, all organs of plants of this genus, except for the stem, flowers and fruits, underwent significant changes: the roots turned into short fleshy fibers of the sucker, sucked to the roots of the host plant, the leaves lost chlorophyll and became small brownish, yellowish or purple scales with the next arrangement … The stem of the broomrape is light brown, yellowish, pinkish or bluish, fleshy, erect, branching or non-branching, with a clavate base equipped with suction cups that penetrate the root tissue of the host plant. The stem height can reach 50 cm or more.
If the soil is heavily contaminated with broomrape seeds and in the presence of the affected plant, one plant may have up to 200 parasite peduncles or more.
Цветки заразихи пазушные, пятичленные, с двугубым немчиком синего, беловатого или фиолетового цвета, с четырьмя тычинками, собраны по несколько десятков в колос или колосовидную метелку. Они способны к самоопылению в том случае, если не было перекрестного, которое осуществляется с помощью заразиховой мухи-фитомизы - Phytomysa orobanchia и шмелей. Завязь - верхняя, одногнездная. Плод - коробочка, раскрывающаяся двумя или тремя створками и содержащая до 2 тыс. семян и более. Семена мельчайшие, длиной 0,2-0,6 мм, шириной 0,17- 0,25 мм, округлые или продолговатые, темно-бурые, с ячеистой поверхностью. На одном растении заразихи их может быть до 100 тыс.
Almost all broomrape has a relatively high specialization. Each species is adapted to parasitizing on a limited range of feeding plants belonging to only one or several specific families, genera and species.
The sunflower broomstick parasitizes mainly on the sunflower; from other plants it affects tomato, tobacco, makhorka, safflower, wormwood, etc. Egyptian broomstick, or melon, affects about 70 species of plants, including potatoes, tobacco, cabbage, tomato, pumpkin. The branched broom, or hemp, infects mainly tobacco, tomato, hemp, cabbage, carrots, melons, etc.
The specialization of the broomrape has changed in the course of evolution, which was facilitated by natural selection and human activity. Along with new forms of plants, in the process of constantly changing relationships between the parasite and the host, new physiological populations and races of the parasite emerged and spread, differing in virulence and the ability to overcome the protective properties of the host plant organism. The number of races of the parasitic species in a given area is determined by the duration of cultivation of the host plant and the diversity of its genotypes. The emergence of new most aggressive races of broomrape leads to the loss of immunity by varieties. For example, in sunflower varieties immune to broomrape, at the site of its introduction into the root of the host plant, swellings are formed, which prevent the further development of the parasite. Such blisters do not occur in the affected varieties.
The development of the parasite is determined not only by the immunological properties of the host plant, but also by the sowing time, soil fertility, the stock of its seeds in the soil, the depth of their embedding, the structure of the root system of the feeding plant, the amount of moisture in the soil, etc. Depending on the biology of the host plant, the broomrape has developed forms of perennial, biennial, annual, and even ephemeral forms. Development, habit and their other features depend on the properties of the feeding plant.
The distinctive features of certain species of broomrape are the morphology of the stem and flower, as well as parasitic specialization.
Sunflower broomrape differs from other broomrape species by its unbranched stem up to 30 cm and more. Her bracts are ovoid, acute; corolla 12-20 mm long, tubular, strongly bent forward, almost not widened at the end, brown in color. The species develops well on cultivated and wild representatives of the Solanaceous and Compositae families. Among them are sunflower, tobacco, makhorka, tomato, perilla, safflower, sea wormwood, Australian wormwood, bitter wormwood, common wormwood, common cocklebur, salt marsh head, odorless chamomile, salt aster. Sunflower broomstick does not infect castor oil plants, soybeans, lallemantia, cabbage, potatoes, mustard.
The Egyptian broomrape, or melon, has a sprawling, branchy stem with a few ovate-lanceolate scales 20-30 cm long. The corolla is 23-27 mm long, tubular-funnel-shaped, significantly widened in the limb. The species mainly infects melons and gourds, as well as makhorka, tobacco, potatoes, sunflowers, mustard, turnips, peanuts, sesame seeds, tomato, cabbage, eggplant and other vegetable, industrial and wild plants (up to 70 species). Does not infect cotton, beets, alfalfa, grapes. Physiological races are known.
Branched broom, or hemp, has a thin, up to 4-5 mm in the middle part, with sparse scales, a stem up to 15-25 cm long, thickened at the base, with a large number (up to several tens) of lateral shoots. The flowers are smaller than those of the above described broomrape species, up to 10-15 mm in diameter. The branched broomstick is less specialized in comparison with other species of the genus. It infects many species of nightshade, Asteraceae, cabbage (cruciferous), pumpkin, etc. Among them are tobacco, makhorka, hemp, hops, cabbage (cabbage, cauliflower, kohlrabi), mustard, turnip, camelina, horseradish, perilla, pumpkin, melon, carrots, dill, coriander, sunflower, lentils, sweet clover, nightshade, peanuts, rope-tree, etc. Does not infect beets, parsnips, lallemantia, parsley, eggplant, pepper. Physiological races are known.
Light, like dust, broomrape seeds are freely carried by the wind, water, stick with the soil to the feet of people, cultivating tools, to the storage organs of plants, and are carried over huge distances by dust storms.
The embryo in the broomrape seed, like in many other parasitic plants, is underdeveloped, not dismembered into a root, stem and cotyledon, but consists of groups of cells surrounded by storage tissue containing nutrients necessary for the seedling until it is sucked to the feeding plant. The optimum temperature for germination of broomrape seeds is 22-25 ° C. They do not germinate at temperatures below 20 ° C and above 45 ° C, some above 50 ° C. The optimum temperature for seed germination of Egyptian broomrape and branched broomrape is higher than that of sunflower broomrape.
Broomrape seeds are able to germinate at any depth of the arable horizon under the influence of root secretions of certain host plant species. If there are no such plants near the broomrape seeds, then they do not germinate, however, they can remain viable for 8-12 years. According to some researchers, with an increase in the concentration of root exudates to a certain limit, the percentage of germinated seeds also increases. In less moistened soil, the concentration of root exudates will be higher; therefore, a particularly strong depletion of sunflower by broomrape is observed in dry years.
A substance secreted by host plants that stimulates the germination of broomrape seeds was found not only in their roots, but also in the leaves and in the bark of the stem (sunflower). This substance is resistant to boiling and drying. It was possible to isolate its crystalline fraction containing a concentrate of stimulating substances.
Root secretions of lettuce, flax, corn, soybeans, perennial legumes (alfalfa, clover, horned broomstick), tomato, earthen pear and others stimulate the germination of broomrape seeds, but since these crops are not susceptible to broomrape, its seedlings cannot find suitable feeding plants die. The use of provocative crops in the fight against broomrape is based on this phenomenon.
The number of germinated broomrape seeds and the energy of their germination depend not only on the root secretions of the host plant, but also on a number of other conditions: on the type of feeding plant, its immunological properties and the concentration of cell sap, on the virulence of the broomrape and the proximity of its seeds to the plant root. host, from the reaction of the environment, temperature and moisture of the soil, etc.
Germination, broomrape adhesion to the roots of the feeding plant and its initial development occur secretly in the soil. When germinating, a slightly twisted sprout emerges from the seed with a clavate thickening at the end, growing in the direction where the concentration of root secretions of the flashing plant is higher. Having touched the root of a plant susceptible to broomrape, the thickening begins to grow, and the rest of the sprout atrophies, turning into a thin thread; then communication with the seed coat is interrupted.
Soon, a thickening at the root of the host plant becomes covered with tubercles, giving it the appearance of a star. One of the haustoria, pushing apart the cells of the parenchyma of the root cortex, penetrates into it and reaches the xylem. Tracheids developing inside the haustorium merge with the conductive elements of the host plant into a single whole so much that it is difficult to find a border between them. At the opposite end of the broomrape, a bud is formed, covered with numerous scales, which later turn into modified leaves. The bud develops into a flowering stem, which brings the inflorescence to the soil surface.
Germination of broomrape seeds scattered in the soil, its suction and development occur gradually as the root system of the feeding plant grows. Therefore, all phases of parasite formation can be observed on the roots of one host plant; from seed germination to capsule ripening. From the moment of germination of broomrape seeds to the appearance of its plants on the soil surface, at least 1.5-2 months pass. It is possible to evaluate sunflower varieties for broomrape resistance without waiting for the broomrape flower stalks to emerge from the soil, by the presence of sucked broomrape on the roots of the host plant.
Broomrape control measures
In protection against flower parasites, a complex of techniques is used.
- prevention of broomrape seeds from entering farms and areas where it does not occur, and thorough cleaning of seeds in infected farms;
- systematic weeding out and destruction of broomrape until it forms seeds and inflorescences to prevent new soil contamination. The weeded broomrape is taken out of the field, burned or buried deep;
- introduction of crop rotations that exclude affected crops for a long period (at least 6-8 years).
Since the broomrape infects various wild plants, the fight against it is an obligatory link in the system of protective measures.
It is possible to free the soil from broomrape with the help of thickened sunflower crops (provocative crops), which cause massive germination of broomrape seeds. During the appearance of the largest number of broomrape flower stalks or at the beginning of its flowering, the culture is harvested for silage. The broomstick does not have time to seed, and when the next crop is harvested, its seeds will be much smaller. For the same purpose, clover or sweet clover are sown. Particularly good results are obtained by introducing new broomrape-resistant and broomrape-tolerant varieties of sunflower and other crops into the culture.
Links to material
- Popkov. K.V. / General phytopathology: textbook for universities / K.V. Popkova, V.A. Shkalikov, Yu.M. Stroykov et al. - 2nd ed., Revised. and add. - M.: Bustard, 2005.-- 445 p.: ill. - (Classics of Russian science).
- Keys to broomrape flora of the USSR (from atlases of fruits and seeds). / E.S. Terekhin, G.V. Shibakina and others - SPb.: Nauka, 1993.-- 127 p.