Orobanche ramosa
(branched broomrape)

O. ramosa does not spread rapidly or aggressively but its introduction in contaminated seed or soil can go undetected, and once introduced it can cause severe damage to important agricultural crops and prove very difficult to eradicate.

O. ramosa occurs naturally in Mediterranean countries in southern Europe, Africa and the Middle East, extending eastwards to India, Pakistan and China, central Asia and southern Russia but has also been introduced to the USA, Cuba, Central America, Australia, West Africa, East Africa, South Africa and Chile.

The risk of introduction from infested areas is high as a result of contamination of crop seed, soil or packaging materials. The minute seeds are extremely difficult to detect and have considerable longevity. Furthermore, this species can establish and propagate on a wide range of non-crop species, allowing it to build up undetected in natural vegetation before attacking host crops.

Orobanche species are listed as prohibited, and/or subject to quarantine, in virtually all countries with developed plant quarantine systems. See, e.g. USDA-ARS (2008) and USDA-NRCS (2008) for regulatory status in the USA.

Most of the weedy Orobanche species are native to the Middle East and are adapted to soils of generally high pH. They occur to some extent in wild vegetation but the weedy species are mostly associated with the crops that they attack. O. ramosa requires relatively high temperatures for optimum germination and growth, and occurs mainly in irrigated crops grown under summer conditions in tropical and sub-tropical climates.

O. ramosa has an especially wide host range, occurring on wild plants in the families Amaranthaceae, Chenopodiaceae, Euphorbiaceae, Capparidaceae, Labiatae [Lamiaceae], Linaceae, Malvaceae, Oxalidaceae, Plantaginaceae, Polygonaceae and Rubiaceae, as well as crops in Alliaceae [Liliaceae], Compositae [Asteraceae], Cannabinaceae, Cruciferae [Brassicaceae], Cucurbitaceae, Leguminosae [Fabaceae], Solanaceae, Rosaceae and Umbelliferae [Apiaceae] (Parker and Riches, 1993). See also Qasem and Foy (2007) for a recent study of host range in O. ramosa.

O. ramosa is attacked by the agromyzid fly Phytomyza orobanchia throughout much of its range and a high proportion of plants may be damaged as a result of larvae reducing seed production and/or mining in the stem, leading to infection by fungi and total collapse. The fly has been used for biological control and was effective in the former Soviet Union for decades. However, this biological control agent gradually became less effective due to the spread of hyperparasites that attack the Phytomyza pupae. See Kroschel and Klein (1999) for a detailed review of this topic. Opius occulisus is the most common hyperparasite in Hungary (Horvath, 1987). Recent attempts to exploit Phytomyza for biocontrol of O. ramosa in Chile have been reported by Norambuena et al. (2001).

Several plant pathogens have been reported specifically to attack O. ramosa (Bedi, 1994; Bozoukov and Kouzmanova, 1994), but none have yet been developed fully for biological control.

To check for the contamination of crop seed stocks, place a crop seed sample (100-400 g) into 1 litre of water containing 0.1% surfactant (e.g. Triton X-100). The water surface can be lightly sprayed with anti-foam. Allow to stand for 10 min, then stir well for 1-2 min. Decant the water (keep the seeds for the next step) onto the top sieve, with openings of 500 µm, which is placed on top of a second sieve with openings of 100 µm. Wash the seeds as above two additional times, decanting the water onto the sieve. On the last wash, dump the entire contents onto the sieve together with the washing water. Using a shower nozzle, thoroughly wash the seeds on the sieve with an additional 5-8 litres of tap water. The presence of Orobanche seeds can be determined on the surface of the lower sieve, with the help of a dissection microscope (Jacobsohn and Marcus, 1988).

After crop establishment, the roots may be carefully retrieved and washed, and inspected for the presence of the typical tubercles, 1-20 mm. Note that the tubercles are easily disconnected from the roots if the root system is pulled out of the soil.

Later in the life of the crop, emerged shoots of O. ramosa will be found, but much damage will by then already have occurred.

Bio-assay of Field Infestation

Flax can sometimes serve as an indicator of field infestation. Flax plants parasitized by Orobanche rapidly develop chlorosis and are considerably stunted. It is therefore used as a reliable tool to pinpoint infected spots in the field (Joel et al., 1990). It can also serve as a tool for infestation diagnosis in pots using soil samples taken from the field, and in experiments where the influence of different treatments on the Orobanche seed bank or on infestation needs to be examined (Joel et al., 1995a).

O. ramosa is distinguished from most other important Orobanche species by the branching habit and the presence of bracteoles. However, there is often confusion with O. aegyptiaca. The latter is typically distinguished by larger flowers, 22-30 mm long and densely hairy connective tissue between anthers, and often the filaments also, while O. ramosa typically has flowers only 15-17 mm long and stamens glabrous or only sparsely hairy. Katzir et al. (1996) have used RAPD techniques to show clear distinction between typical populations of O. aegyptiaca and O. ramosa but intermediates with flowers about 20 mm long and moderately hairy anthers can be difficult to assign and leave some doubts about the full distinctness of these two species.

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.

Prevention

Phytosanitary Measures

Most countries prohibit entry of major parasitic weed species, including Orobanche spp.

Phytosanitation is aimed at preventing the spread of viable seeds by 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 of Orobanche-infested plants (Jacobsohn, 1984). One should also avoid the use of Orobanche-infested crop seeds. Panetta and Lawes (2007) discuss the intensive monitoring that is needed in the course of an eradication programme such as that being implemented in Australia.

Control

Cultural control

Hand-weeding of emerged stems is too late to prevent crop damage but may be worthwhile where infestations are still light, to prevent or reduce future infestations. The stems should immediately be removed from the field to preclude seed shed after pulling. Unfortunately it is often difficult to see O. ramosa shoots under the canopy of a dense tomato crop.

Trap crops may be used to promote germination of Orobanche seeds in soil, without themselves supporting parasitism, in order to deplete the seed reserve. Examples of trap crops for O. ramosa include flax, Phaseolus bean, sorghum, maize and cucumber (Parker and Riches, 1993). There are few examples of the fully successful use of this principle, but it should be considered in any integrated control approach.

Soil solarization, based on mulching moist soil with polyethylene sheets for several weeks under solar irradiation, can provide excellent levels of control of Orobanche seeds in the upper soil layers where temperatures are high enough (Jacobsohn et al., 1980), and this has been confirmed in a number of studies involving O. ramosa (see Parker and Riches, 1993; Mauromicale et al.,2005).

Kebreab and Murdoch (1999b) showed that seeds maintained at high moisture and high temperature, lose viability relatively rapidly. This could explain the success that has been occasionally reported from prolonged flooding or water-logging (e.g. Mohamed Ahmed and Drennan, 1994), but a period of at least 6 weeks may be needed.

High levels of nitrogen have been reported to reduce O. ramosa in tomato and tobacco (Parker and Riches, 1993; Demirkan and Nemli, 1994). In some cases good crop yield increases have been recorded (Abu-Irmaileh, 1985) but in others the crop has been damaged and care is needed to maintain the correct balance between nitrogen and phosphorus. Nitrogen as ammonium is much more effective than as nitrate (e.g. Westwood and Foy, 1999). Haidar and Sidahmed (2006) report the successful use of chicken manure for control of O. ramosa in aubergine and potato in Lebanon.

Host-Plant Resistance

While host-plant resistance has been very important in the control of Orobanche cernua in sunflower, there has been little success in finding resistance to O. ramosa. Screening of tobacco and tomato varieties against O. ramosa or O. aegyptiaca have demonstrated some variations in susceptibility (see Parker and Riches, 1993; Qasem and Kaswari, 1995; Mariam and Suwanketnikom, 2004), but there are no reports of the successful application of these results.

Biological control

The fly Phytomyza orobanchia has been used for biological control of Orobanche spp., including O. ramosa, and was effective in the former Soviet Union for decades, using special rearing and inundative release techniques. However, this became less effective due to the spread of hyperparasites. See Kroschel and Klein (1999) for a detailed review. New attempts to exploit P. orobanchia for biocontrol of O. ramosa in Chile are reported by Norambuena et al. (2001).

Several Fusarium spp. and other plant pathogens have been reported specifically to attack O. ramosa (Bedi, 1994; Bozoukov and Kouzmanova, 1994), but none have yet been developed fully for biological control.

Chemical control

Alternatives to now banned soil fumigation methods including metam-sodium and dazomet may provide good control but methods of use are critical and best results are normally achieved with soil coverage by plastic (see Parker and Riches, 1993). Recommended doses of these compounds are usually very high and costly but much lower doses have been reported by Chalakov (1998) to be effective in Bulgaria, perhaps resulting from a germination-stimulatory effect and death by suicidal germination.

Among the few herbicides to have shown adequate selective control of O. ramosa, imazethapyr has been reported effective in potato, but three applications are needed at 2-week intervals (Kleifeld et al., 1998). This was not safe when used on tomato. The sulfonylurea herbicides chlorsulfuron, rimsulfuron and triasulfuron have shown some selectivity against both O. aegyptiaca and O. ramosa in tomato, but application methods are critical, preferably through drip irrigation (e.g. Kleifeld et al., 1996; Vouzounis and Americanos, 1998; Goldwasser et al., 2001). Simple recommendations are not often possible but Goldwasser et al. (2001)indicate that rimsulfurondoes now have approval for used on commercial crops of potato. Glyphosate at low doses post-emergence has likewise shown some selectivity in both tomato and tobacco but the margin of safety is too small for reliability. In potato, a low dose of rimsulfuron followed by 3 low-dose applications of glyphosate gave best results (Hauidar et al., 2005).

A range of volatile vegetable oils was also effective against O. ramosa in a greenhouse experiment (Solymosi, 1998).

The use of transgenic crops engineered with target-site herbicide resistance is 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 has been achieved without affecting the crop or its yield (Joel et al., 1995b; Nandula et al., 1999). Comparable results can be expected against O. ramosa.

For the latest review of control methods for Orobanche spp, including O. ramosa, see Joel et al. (2007).

USA: International Parasitic Plant Society, James H. Westwood, Ph.D. Associate Professor Virginia Tech Department of Plant Pathology, Physiolo, 401 Latham Hall Blacksburg, VA 24061-0390, http://www.parasiticplants.org/default.asp

10/04/2008 Updated by:

Chris Parker, Consultant, UK