Root parasitic weeds are a serious problem in many agricultural production systems. Unlike "normal" weeds that merely compete with the crop plants for nutrition and harbour diseases, root parasitic weeds damage the crops by attaching their own roots to the roots of the crop plant and taking their nutrition and water from it. They are especially hard to control because they cannot be treated as a separate plant and also they inflict much damage before emerging above ground. Of all root parasites,Orobanche (broomrape) and Striga (witchweed) see weeds
cause most damage to agricultural crops.
Broomrape (Orobanche) is a genus of over 200 species of parasitic herbaceous plants in the family Orobanchaceae, native to the Mediterranean region (North Africa, the Middle East and Southern Europe) and Western Asia. Their range extends to similar climates in Africa, Asia, Australia, and North and South America where they cause significant crop damage. It is broadly estimated that it affects over a million hectares of agricultural land with average yield losses as high as 40 per cent in some areas. The extent of economic damage caused by Orobanche is unknown
The broomrape plant is small, from 10-60 cm tall depending on species. It is best recognized by its yellow-to-straw coloured stems completely lacking chlorophyll, bearing yellow, white or blue, snapdragon-like flowers. The flower shoots are scaly, with a dense terminal inflorescence
(spike) of 10-20 flowers in most species. The leaves are merely triangular scales. The seeds are minute, tan-to-brown, and blacken with age. These plants generally flower when weather conditions are cool to moderately warm. When they are not flowering, no part of these plants is visible above the surface of the soil.
As they have no chlorophyll, they are totally dependent on other plants for nutrients. Broomrape seeds remain dormant in the soil, often for many years, until stimulated to germinate by certain compounds produced by living plant roots. Broomrape seedlings put out a root-like growth, which attaches to the roots of nearby hosts. Once attached to a host, the broomrape robs its host of water and nutrients.
Members of the genus Orobanche parasitise roots of broad-leaf plants and cause severe damage to vegetables and other field crops. Some species are relatively specific in their host selection, while other species have a very wide host range. The most important species and their hosts are:
O. aegyptiaca (Egyptian broomrape): Tomato, potato, tobacco, eggplant, bell-pepper, pea, vetch, faba bean, carrot, celery, parsley, cumin, cabbage, cauliflower, rape, mustard, turnip, hemp, sunflower, spinach. In some areas, e.g. southern Russia, sweet melon and water melon are also hosts. Also parasitises ornamentals like Chrysanthemum and Gazania
O. ramosa (branched broomrape or hemp broomrape): Hemp, potato, tobacco and tomato; also on groundnut, cowpea and chilli pepper. O. ramosa was also recorded on aniseed, basil, fennel, and a range of ornamental species.
O. crenata (bean broomrape): An important pest of legumes: faba bean, pea, lentil, chickpea, vetch and clover. Also attacks carrot, parsley, celery, cumin, safflower, Carthamus, eggplant, tomato, lettuce, geranium and verbena.
O. cernua (Nodding broomrape): Mainly attacking solanaceous crops: tomato, potato, tobacco and eggplant.
O. cumana (sunflower broomrape): An important pest of sunflower, also parasitizing safflower. May in certain cases parasitise tomato, when growing in soil previously cropped with sunflower. It is also known to attack caraway (Carum carvi).
O. minor (Common broomrape): It attacks red and white clover, lucerne, tobacco, carrot, lettuce, sunflower and many other crops and ornamentals on a local basis.
In order to determine the host it is necessary to check which plant's roots are indeed connected to the parasite. There are some erroneous reports based on observations of neighbouring plants rather than of connected hosts. Sometimes the detection of a host is very difficult, especially when a field is used simultaneously for more than one crop, or when the field is loaded with weeds.
Lycopersicon esculentum (tomato), Solanum tuberosum (potato), Nicotiana tabacum (tobacco), Solanum melongena (aubergine), Capsicum annuum (bell pepper), Pimpinella anisum (aniseed), Pisum sativum (pea), Coronilla varia (crown vetch), Vicia faba (broad bean), Arachis hypogaea (groundnut), Vigna unguiculata (cowpea), Lens culinaris ssp. culinaris (lentil), Lathyrus sativus (grasspea), Trifolium (clovers), Daucus carota (carrot), Apium graveolens (celery), Petroselinum crispum (parsley),Cuminum cyminum (cumin), Brassica oleracea var. capitata (cabbages), Brassica oleracea var. botrytis (cauliflower), Brassica napus (rape), Brassica nigra (black mustard), Brassica rapa (turnip), Cannabis sativa (hemp), Papaver (poppies), Helianthusannuus (sunflower), Chrysanthemum (chrysanthemum), Carthamus tinctorius (safflower), Lactuca sativa (lettuce), Cucumis melo (melon), Citrullus lanatus (watermelon), Spinacia oleracea (spinach), Foeniculum vulgare (fennel), Carum carvi (caraway),Ocimum basilicum (basil), Geranium (cranesbill), Verbena
Infested plants exhibit symptom
s such as leaf yellowing, water stress, stunting and wilting
Affected plant stages
Vegetative, growing stage and flowering stage.
Affected plant parts
Whole plant, leaves and roots.