Orobanche - The Broomrapes

INTRODUCTION

Orobanchaceae, the broomrape family, comprises approximately 150 species in 17 genera. Four genera represented by four species occur in the southeastern U.S. (21). A majority of the genera and about 90% of the species in Orobanchaceae are Old World natives. The family is primarily' one of the northern warm and temperate zones. Only about 10% of the species occur in the tropics; only one species reaches the arctic (21).

A few species in Orobanchaceae are used in folk medicine (21). Several Old World species are widely distributed weeds. The genus Orobanche accommodates about 60 species of unbranched parasitic herbs, without chlorophyll (21). The broomrapes are variable in color, ranging from yellowish-brown and reddish-violet to purple, blue, and orange (13). Species are found over much of the world, including most of Europe, Asia, and North America, and the majority of species grow only on one or two host plants (13).

The parasitic broomrapes live directly on their hosts by attaching strong haustoria to their roots, penetrating the tissues, and absorbing the food gathered by the host plants for their own development. The chief weedy species in the U.S. were imported in clover (Trifolium spp.), hemp (Cannabis sativa L.), and tobacco (Nicotiana tabacum L.) seed, and in like manner, spread to several states (1).

In 1753, Linnaeus erected the genus Orobanche in his Species Plantarum. He derived the name from the Greek orobos, a vetch, and ancho, to strangle, referring to the effect the plants have on their hosts (6).

The name broomrape is a rendering of medieval Latin Rapum genistea, broom knob or tuber; rapum being a knob or lump formed by the roots of trees, and genista, a broom. Therefore this name is not of popular origin (20). It was first applied to great broomrape, Orobanche major, the Rapum genistae of Lobel and Crenate broomrape seeds require about a week of stimulation to start germination, which occurs about one week before the host plant flowers. A substance produced by the roots of mature host plants encourages broomrape seed germination (7). Consequently, seeds may germinate at a distance up:to 1 cm from the host plant, but only those at a distance of 2 to 3 mm actually cause infection.

SEED SIZE AND LONGEVITY

Most weeds have seed weights ranging from about 0.1 g per 1000 seeds to about 3.0 g per 1000 seeds, and the lighter seeds are considered to have a decided advantage in dissemination (9). On this basis, broom-rape is a colossus. Some species bear seeds in prodigious numbers and of near-microscopic size; e.g., seeds of small broomrape (Orobanche minor Sm.) are 0.2 to 0.3 mm long and 0.2 mm wide, and 0.1 thick (9). Additionally, 1000 seeds of O. elatior weigh 0.0049 g and one plant produces 270 000 seeds. O. picridis produces from 94000 to 116000 seeds per plant and 1000 seeds weigh 0.0029 g (8).

Broomrape seeds are widely disseminated by the winds and, though minute, are very long-lived, having been known to survive in the soil for 13 yr (1). They are capable of lying dormant for several years and germinating in succession. Greenhouse experiments conducted for 14 yr showed that broomrape seeds retain their vitality when buried in soil for 8 yr. Other researchers report the time as at least 10 yr, the varia-tion probably resulting from different conditions (17).

For survival, parasitic plant seedlings must quickly find a suitable host plant (12). To germinate, broomrape seed requires exposure to a biochemical exudate produced by the root of the host plant a phenomenon highly advantageous to the parasite. The radicle of Orobanche species grows chemotropically toward host plant roots. This germination feature probably is highly evolved and acts to enhance seeding survival (18).

SEEDLING DEVELOPMENT

Broomrape seedlings are trivial, light-colored, nearly transparent threads, lacking roots and chlorophyll. Although they have no difficulty in penetrating the soil, they are unable to draw sustenance from it (11). If a suitable host is not found soon after germinating, the seedlings shrivel and die.

Upon germination, broomrape seed develops a small radicle which penetrates a fine rootlet of the host and becomes firmly connected with it. The radicle immediately begins using nutrients from the host, and stores these as a starchy reserve in the upper part of the radicle, causing it to become distended. It gradually forms a nodule on the host root (1). As the nodule increases in size, small prominences emerge on its surface. The prominences develop into roots, which surround the nodule closely, and penetrate the host roots in other places, thus becoming attached at several points. The large swelling develops into a shoot, which ultimately elongates to form the above-ground portion of the plant. The greater part of the broomrape's life is spent below ground, as nodule development into a fair sized bulb-like structure, necessary for the development of aerial shoots, requires several weeks (1).

A few host plants stimulate the germination of broomrape seeds without themselves being parasitized, including chili and bell peppers (Capsicum annuum L.), coat buttons (Tridax procumbens L.), and hairy beggar-ticks (Bidens pilosa L.) (9). When broomrape seed occurs close to the roots of such species, 13 to 15% of its seeds germinate but do not d1-1velop haustorial connections. Peppers grown in rotation with tobacco tend to reduce broomrape seed infestations in the soil (9).

THERAPEUTIC USES OF BROOMRAPE

During the 16th and 17th centuries, great broomrape ( O. major, synonym O. rapum-genistae) of Europe and Russia was prescribed as a medicinal herb throughout most of Europe "as a remover of stone in the bladder and kidneys-and as a provoker of lustry urine and was usually administered decocted in wine. Reportedly it was bitter tasting and very astringent (13). Applied externally, the juice was regarded as "a singular remedy" in the treatment of "old green wounds-the runnings of ulcers and sores-malignant and scabby ulcers, those that be hollow also" and "for fretting sores." The decocted flower spikes were used as a wash for "cleansing the skin" and "for freckles, black or blue spots or pushes thereof' (13).

Great broomrape generally attaches itself to the roots of broom (Cytissus sp.) or gorse (Ulex europaeus L.). This stout leafless club-like perennial grows 70 to 100 cm tall. Its honey-brown stems produce a dense spike of yellowish, purple-tinged flowers from May to July. Although very distasteful, the entire plant of cancer root (O. virginiana L.), after "yielding its virtues" to water or alcohol, was administered "with benefit in fluxes and diarrhea and certain afflictions to the bowl," and as a useful "application to obstinate ulcers, aphthous ulcerations," while locally "applied to wounds it prevented or arrested the process of mortification" and "arrested gangrenes" (13). Now cancer root is rarely used in medicine, probably because of its nauseating taste.

Cancer root, a North American native, is found from Wisconsin, south to Florida and Louisiana. Despite its common name, the species was never actually considered a cancer cure. Another common name is beech-drops, because it is parasitic on roots of European beech (Fagus sylvatica L.). During the 19th century it was prescribed as "an eminent astringent") (13). Its smooth slender branched stems reach 30 to 50 cm tall, and in August and September produce small clusters of whitish or whitish-purple flowers.

INTRODUCED NEW WORLD WEEDS

Branched broomrape (Orobanche ramosa, L.), an introduction from Europe, is also known as tobacco broomrape and hemp broomrape. It bears erect, slender, many-branched brown or straw-colored stems, 10 to 20 cm high, and produces yellow or pale blue flowers in a spike. Its one-celled capsules are many seeded. Branched broomrape occurs primarily in Illinois, Kentucky, Tennessee, North Carolina, and New Jersey. It is parasitic on tobacco and tomatoes (2).

At times, branched broomrape causes severe yield loss in tomatoes in California (9), where it originally spread in seed-infested soil retained on farm implements. Its hosts include redroot pigweed (Amaranthus retroflexus L.), shepherd's-purse (Capsella bursa-pastoris (L.) Medicus), nightshade (Solanum spp.), and spiny cocklebur (Xanthium spinosum L.). These weeds may ensure the survival of branched broomrape. This is important in California, where the parasite often grows in abundance on weeds in harvested fields of cucumber (Cucumis sativus L.) and along unpaved roads separating old sugarbeet (Beta vulgaris L.) fields-two nonhost crops, thus surviving from year to year (9).

Small broomrape, Orobanche minor, another European introduction, also is known as lesser broomrape, chokeweed, herb-bane, clover devil, devil's root, and hellroot. It ranges from New Jersey southward to North Carolina, and is found locally in a few places in interior states. It inhabits clover fields but it also parasitic on tomato and tobacco (16).

Small broomrape was brought inadvertently to the U.S. in clover seed. The parasite grows larger than its host and its presence in any abundance destroys the crop. Its seed has long vitality in the soil (15). Holdsworth and Nutman (5) observed that small broomrape does not initiate flowers unless the host has reached the flowering state. However, flowering in several strains of Orobanche occurs on purely vegetative hosts (10). Small broomrape can infest clover 7 yr after the last clover crop was grown (16).

A NATIVE BROOMRAPE

The native Louisiana broomrape, O. ludoviciana Nutt., inhabits sandy soils, chiefly in the Mississippi Valley (16). However, it has a wide range-Illinois to Saskatchewan, south to Texas, and west to California and Arizona. It is parasitic on several wild plants, but has also attacked tobacco. In general habit and appearance it is similar to branched broomrape. Its flowers are borne in a dense spike. Louisiana broomrape has a wider range than its immigrant relatives, but it is of economic importance only in tobacco growing areas (1). In spring and early summer the Pah Ute Indians ate the thick, white fleshy bases of Louisiana broomrape (13).

PREVENTION

The best control remedy is prevention, i.e., sowing clean seed. Broomrape seeds are among the tiniest in existence, so they are easily removed from large-seeded crops (1).

Cultural methods generally involve the use of a susceptible species called a "trap" crop, which is alternated in rotation with the desirable crop species (9). Badly infested fields should be planted for 2 or 3 yr with crops that are not parasitized by the weed (2).

For centuries, clean cultivation followed by hand weeding or hoeing broomrape plants before they produced and disseminated seeds was advocated for control. This was especially important with new infestations. The attachment with the host roots is not very firm, and the whole parasite breaks away readily. It is necessary to grub broomrape plants systematically for several years until the dormant seeds have all developed (15). The survival of only a very few plants is sufficient to reinfest a large area.

LITERATURE CITED

1. Brenchley, W. E. 1920. Weeds of Farm Land. Longman, Green and Co., London.

2. Georgia, A. E. 1942. A Manual of Weeds. The Macmillan Co., New York.

3. Giliarov, M. S. 1943. Orobanche ramosa on kok-saghyz in Russia. Biol. Abstr. 17, 3021. 

4. Henslow, G. 1898. Orobanche speciosa. Gardener's Chron., Ser. 3, 24: 89.

5. Holdsworth, M. and P. S. Nutman. 1947. Flowering responses in a strain of Orobanche minor. Nature 160:223-234.

6. Jaeger, E. C. 1944. A Source-book of Biological Names and Terms (2nd Ed.). Charles C. Thomas, Springfield, IL.

7. Kardy, A.E.R. and H. Tewfic. 1956. Seed germination in Orobanche crenata Forssk. Svensk Bot. Tidsk. 50:270-286.

8. King, L. J. 1952. Germination and chemical control of the giant foxtail grass. Contrib. Boyce Thompson Inst. 16:469-487.

9. King, L. J. 1966. Weeds of the World. Interscience Publishers, Inc., New York.

10. Kribben, F. J. 1952. Die blutenbildung von Orobanche. Ber. Deutsch. Bot. Ges. 64:353-355.

11. Kuijt, J. 1969. The Biology of Parasitic Flowering Plants. Univ. of California Press, Berkeley.

12. LeStrange, R. 1977. A History of Herbal Plants. Angus and Robertson, London.

13. Marudagan, D. 1950. Note on Orobanche cernua Loefl. Current Sci. 19(2):64--65.

14. Menscher, W. C. 1948. Weeds. The Macmillan Co., New York.

15. Pammel, L. H. 1912. Weeds of the Farm and Garden. Orange Judd Co.,

New York.

16. Passerini, N. 1910. Duration of vitality of seeds of Orobanche crenata. Atti R. Accad. Econ. Agric. Georg. Firanze, Ser. 7, 5(1):1-7.

17. Radosevich, S. R. and J. S. Holt. 1984. Weed Ecology. John Wiley & Sons, New York.

18. Rao, M. K. 1919. Orobanche in tobacco. J. Mysore Agric. Exp. Univ. 1(3):18-20.

19. Simpson, J. A. and E.S.C. Weiner. 1989. The Oxford English Dictionary, 2nd Ed. Clarendon Press, Oxford.

20. Thieret, J. W. 1971. The genera of Orobanchaceae in the southeastern United States. J. Arnold Arboretum, 52:404-413.