Orobanche

(broomrape), a genus of perennial or annual herbaceous nonchlorophyll plants of the family Orobanchaceae. The stems grow to 50 cm and taller; they are light brown, yellowish, pink, or bluish; fleshy; and simple or branching, with a claviform base and squamiform leaves. The roots have evolved into short, fleshy fibers (haustoria), which attach themselves to the roots of a host plant. The flowers are in spicate inflorescences; they are pentamerous, with a bilabiate blue, whitish, or violet corolla and four stamens. The flowers are pollinated by bumblebees or flies and self-pollination is also possible. The ovary is superior and unilocular. The fruit is a polyspermous pod, with up to 2,000 seeds. The dark brown round or elongated seeds are very small. They are easily carried by wind or water or in soil that sticks to feet, soil-cultivating tools, or plant organs. The seeds germinate at a soil temperature not lower than 20°C, and as a rule are activated by the root secretions of the host plant or by a fixed soil acidity (pH 5.3–5.8). The seeds keep their vital capacity for up to eight to 12 years.

There are more than 150 species of Orobanche in the temperate and subtropical regions. In the USSR there are about 80 species, mainly in the southern areas. Many species of Orobanche are dangerous parasitic weeds that are difficult to eradicate. Each species of Orobanche is linked with a strictly determined group of host plants. The most harmful species are O. cumana, which parasitizes sunflowers, tomatoes, and tobacco; branched broomrape, or O. ramosa, attacking primarily hemp and tobacco;O. brassicae, orO. Mutelii, attacking cabbage, tomatoes, and tobacco;O. aegyptiaca, attacking watermelon, muskmelons, and pumpkins; and O. lute a, attacking alfalfa and clover. Some species, for example, O. cumana, O. ramosa, and O. aegyptiaca, have physiological races and populations that differ in the degree of their harmfulness. Members of the genus Orobanche deprive their hosts of water and nutritive substances and poison them with the products of their parasitic metabolism. Many plants wither and die. The best prophylactic measure against broomrape is to raise plant varieties that are immune to it. Other measures are crop rotation, in which crops that are attacked by broomrape should not be returned to a site more often than every eight or nine years; working of the soil to a depth of more than 20 cm; early sowing of agricultural crops that are attacked by broomrape; weeding; rooting out of the broomrape before it goes to seed and making sure it is destroyed; provocation sowing of crops like lettuce, soybeans, maize, and flax, which stimulate the germination of broom-rape seeds but are not hosts; provocation sowing of early maturing crops that are attacked by broomrape and harvesting them before the broomrape seeds mature; and treatment of the crops with herbicides. Biological methods of control use the fungus Fusarium orobanche and Phytomyza orobanchia flies.