Orobanche minor
(common broomrape)

Orobanche minor is not a highly invasive plant but its introduction could result in a significant impact on certain crop species, especially forage legumes and tobacco.

Annual
Herbaceous
Parasitic
Seed propagated
Succulent

O. minor has a wide natural distribution through most of Europe (excluding the extreme north), North Africa and western Asia. It has been introduced further afield in the USA, S. America, Australia, New Zealand and southern Africa. It is less clear whether occurrence in the highland tropics of eastern Africa is natural or introduced.

The risk of introduction is not high, as O. minor is not an especially frequent weed of crops and, even when it is, the chances of contamination of crop seed is moderate or low. However, any contamination that does occur may have serious consequences where it contravenes plant quarantine regulations.

O. minor is associated mainly with disturbed areas, crops, roadsides and waste places on a wide range of soil types in temperate and highland tropical areas. There is some tendency to association with alkaline soils but the local importance of O. minor on tobacco suggests that this is not a strict limitation. Although the other main group of root parasites, Striga spp., are favoured by low soil nitrogen, this is not generally true for Orobanche spp. Yoneyama et al. (2001) even report increased stimulation of O. minor germination by exudates from roots of Trifolium pratense at increasing levels of nitrate, although ammonium could be inhibitory. Phosphate also greatly reduced stimulant exudation, in keeping with field observations of reduced O. minor infestation in Trifolium subterraneum at higher phosphate levels (Southwood, 1971). Westwood and Foy (1999) also showed ammonium nitrogen to be directly inhibitory to radicle elongation in a number of Orobanche species, O. minor being the most susceptible to this effect. O. minor may occur in some irrigated crops but is not common in naturally wet habitats.

O. minor parasitizes numerous species in a wide range of plant families, predominantly Leguminosae [Fabaceae], Umbelliferae, Solanaceae and Compositae [Asteraceae] but also many others including Malvaceae, Geraniaceae and Rosaceae. Musselman et al. (1981) found some evidence for the existence of biotypes with differing specificity; a population from a lettuce host parasitized tobacco and Trifolium species as well as lettuce, whereas populations from Trifolium species failed on lettuce and were very slow to emerge on tobacco. However, seeds of O. minor var. minor taken from plants parasitizing Eryngium maritimum were able to parasitize Hypochaeris radicata, Trifolium hybridum and T. pratense (Hipkin, 1992).

Where morphological variants are associated with particular host species it is not known whether the morphology is affected by the host, nor whether these variants are truly host-specific.

By far the most significant insect enemy of O. minor is the dipteran Phytomyza orobanchia, associated with O. minor through most if not all of its natural range. Only a selection of countries are indicated in the Table. Kroschel and Klein (1999) indicate its occurrence in most of Europe, the Mediterranean, the Balkans and Central Asia. Reductions of total seed production resulting from natural infestations range from 11 to 90% and it has been exploited for biological control (see 'Control' section).

Orobanche species are affected by a wide range of fungal pathogens and many of those recorded by Linke et al. (1992) on O. crenata are likely to also occur on O. minor, in addition to Aspergillus niger and Alternaria alternata which were recorded on O. minor. The complete collapse of plants sometimes observed after attack by Phytomyza is often the result of fungal infection associated with the physical damage caused to the stems by Phytomyza larvae.

O. minor is distinguished from the important weedy species O. ramosa and O. aegyptiaca by lack of branching and the absence of bracteoles. There is greater difficulty distinguishing it from other members of the section Osproleon, especially from dried material. O. crenata is close to O. minor in many respects but is larger in most of its parts and its flowers are almost invariably over 20 mm long and over 12 mm across. In O. cernua (including O. cumana), the flowers are similar in size to those of O. minor, but when fresh they are clearly distinguished by the contrast between a white tube and deeply coloured lobes. Dried material shows less contrast, but dissection of the corolla tube shows the stamens of those two species to have glabrous filaments inserted at least 5 mm from the base of the tube.

Benharrat et al. (2000) have recently used molecular techniques to confirm the morphological distinctions between O. minor and the closely related taxa O. amethystea, O. loricata and O. hederae. For a full monograph see Beck-Mannagetta (1930). For keys which include all the main species of concern see Chater and Webb (1972) and Parker and Riches (1993).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Cultural Control

Infestation may be prevented by phytosanitation, aimed at preventing the spread of viable seeds. This may involve minimizing the movement of infested soil by farm machinery and vehicles, preventing grazing on infested plant material, treating manure (e.g. composting) and avoiding the use of hay made from Orobanche-infested plants (Jacobsohn, 1984). The use of Orobanche-infested crop seeds should also be avoided.

Trap crops and catch crops are used in the control of some other Orobanche species. These stimulate germination of Orobanche in the soil, in order to deplete the seed reserve. Trap crops promote Orobanche seed germination but do not support parasitism; catch crops support parasitism but are destroyed prior to Orobanche flowering. Examples of trap crops for Orobanche include flax (Ramaiah, 1987), mung beans, maize and sorghum (Foy et al., 1989). A catch crop that has been effective in Egypt is berseem (Trifolium alexandrinum), harvested repeatedly for forage to prevent full development and seeding of the parasite (Al-Menoufi, 1994). Kropac (1973) has also reported that a catch crop of Trifolium repens and Lolium multiflorum was effective in eliminating seeds of Orobanche spp., including O. minor. Though these methods have in some cases proved successful, the use of trap and catch crops has limitations for two reasons: (a) local strains of the parasites may differ in their response; (b) it takes several years of trap or catch cropping to reduce parasite seed populations to non-damaging levels.

Frater (1975) proposes deep ploughing as a control measure, depending on the observation that infestation on tobacco in New Zealand arises from shallow soil layers.

Fertilization, especially phosphate and ammonium nitrogen, can help to suppress O. minor (Yoneyama et al., 2001). Westwood and Foy (1999) show that the levels of ammonium required to inhibit O. minor are non-toxic to a range of likely host crops.

Host-plant resistance is an important approach to the control of several other species of Orobanche, but there is no known instance of the selection of crop varieties resistant to O. minor.

Mechanical Control

Hand-pulling of Orobanche flowering stems can be useful, preferably at an early stage before maximum damage has been caused, but stems should immediately be removed from the field as they may continue developing flowers and spreading seeds even without being connected to the host. Hand-weeding is very important, especially when only a few Orobanche plants develop in a field. This can prevent further spread of the parasite and avoid damage.

Chemical Control

Soil fumigation with metham sodium [metam] has been used with moderate success against O. minor in New Zealand (James and Frater, 1977).

There are few herbicides that have an adequate margin of selectivity against Orobanche species and none have been developed specifically for control of O. minor. Glyphosate can be used for control of O. crenata in faba bean and some other crops if applied at low rates (Garcia-Torres, 1994). The imidazolinone herbicides have shown promise applied in various ways for control of Orobanche spp. in legumes and sunflower, especially imazethapyr pre-emergence, post-emergence or as seed dressings in legumes, and imazapyr in lentil and sunflower (e.g. Garcia-Torres and Lopez-Granados, 1991a, b; Garcia Torres et al., 1995, 1996, 1998). A number of these options are likely to be suitable for control of O. minor in the relevant crops, but these compounds will need to be used with care to avoid the development of herbicide-resistant populations of Orobanche. For a broad review of control options for Orobanche spp., see Parker and Riches (1993).

The use of transgenic crops engineered with target-site herbicide resistance is perhaps one of the most promising solutions for Orobanche infestation in many crops. Using glyphosate on transgenic oilseed rape, and chlorsulfuron and asulam on tobacco, complete control of O. aegyptiaca was achieved without affecting the crop or its yield (Joel et al., 1995). These techniques have yet to be applied to the control of O. minor, but when the relevant herbicide-resistant crops become available they should prove valuable.

Biological Control

The broomrape-fly Phytomyza orobanchia was widely used for Orobanche control in the Soviet Union and some East European countries in the 1960s and 1970s (Girling et al., 1979). Infested Orobanche shoots were collected during the growing season, dried and stored under controlled temperature and humidity over the winter and released from specially constructed 'phytomyzaria' which allowed separation and destruction of the smaller hyperparasites as they emerged. The topic has recently been reviewed in some detail and there has been some renewed interest in its exploitation (see Kroschel and Klein, 1999; Klein and Kroschel, 2002). These authors provide detailed reviews of the biology of the insect, its very large number of hyper-parasites (at least 24 are listed) and the early elaborate systems for its exploitation in eastern Europe, where success was often over 90% in terms of reduced Orobanche seed production in the treated fields, but problems had arisen from interference by hyperparasites and also from insecticide use. They emphasise that repeated releases are needed over several seasons to overcome the problem of the long-lived seed bank. Recent attempts at introduction of P. orobanchia to Chile as a classical biocontrol agent against introduced infestations of O. minor and O. ramosa are showing promise (Normabuena et al., 1999, 2001).

Orobanche-specific forms of Fusarium oxysporum have shown promise for control of some other Orobanche spp. in tobacco and in sunflower (Parker and Riches, 1993). Ulocladium atrum has also appeared promising against O. crenata (Linke et al., 1992), but neither of these agents has yet been developed for practical use.

Integrated Control

Ideas for integrated control of Orobanche species are reviewed by Pieterse et al. (1992) and by Parker and Riches (1993). Approaches for possible inclusion in an integrated control programme include general techniques of crop rotation, flooding, fumigation, solarization, prevention of seeding and biocontrol, as well as more crop-specific methods including crop resistance, herbicides and time of planting.