Orobanche (broomrape)

Orobanche causes severe direct damage to a variety of crops. The extent of the damage depends on the specificity of host/parasite relations, and on the extent of infestation. Orobanche is an obligatory root parasite that attacks developing roots both of young and mature host plants. The parasite can therefore be seen only on host roots, and its flowering stems emerge from soil in the vicinity of host plants. The damage, on the other hand, can be seen in any of the host organs.Early infection is characterized by the attachment of small tubercles of the parasite to host roots. The young parasite is spherical, brownish-yellow, and a few millimetres long. More advanced parasites (1 cm) bear roots of various lengths, often orange or brown in colour, that form a crown around the infection site, on the host root surface. Orobanche roots are fragile and often rather short, a few millimetres (O. cumana) up to a few centimetres (O. crenata). Infection is dependent on the season in some species, i.e. O. crenata needs cold winter temperatures, whereas O. cernua is a summer parasite in most countries. Infection is possible only after Orobanche seeds are triggered to germinate by substances released from host roots or by neighbouring plants. Therefore some hosts are infected during limited developmental stages, only when their roots release stimulants. Orobanche infection can kill host seedlings. Infection at later stages affects host development, its flowering and fruiting, causing chlorosis or inhibiting growth. In many cases the number of host flowers is reduced due to Orobanche infection, or pollinated flowers are shed, or fruits drop before ripening, or fruits do not grow to their optimal size. Orobanche limits sugar accumulation in developing carrot roots.Hibberd et al. (1996) show that the effects of O. aegyptiaca in reducing biomass of tomato and tobacco are attributable to the amount of dry matter diverted from host to parasite. Stem, fruit and roots are most reduced, while leaf area is relatively unaffected.

Introduction

There is so far no economical, feasible or universal means of controlling any of the weedy species of Orobanche. There are a few traditional methods of control that are relatively ineffective in reducing the immediate damage, but are important for limiting further increase of Orobanche seed bank in soil and preventing its spread.

Sanitary Methods

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.

Cultural Control

Hand weeding
Orobanche flowering stems should be weeded as early as possible. The stems should immediately be discarded and must not remain in the field because they can 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.

Avoidance
Delaying the sowing of winter crops may decrease O. crenata infestation and reduces the damage it causes. In a similar manner, early sowing of sunflower reduces infestation by O. cumana (Garcia Torres, 1994).

Another way to avoid infestation is to grow non-host crops in the infected field. It is necessary to know the host preferences of the particular Orobanche population in the field, and it should be remembered that Orobanche seeds remain viable in soil for many years, sometimes up to 15-20 years.

Trap crops and catch crops
Both are used to promote extensive germination of Orobanche in 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, 1991). Even though this method sounds promising, and in some cases proved successful to a certain extent, the use of trap and catch crops has not gained an important role in the control of root parasites anywhere in the world, 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.

Solarization
Soil solarization, based on mulching soil with transparent polyethylene sheets for several weeks under solar irradiation, kills Orobanche seeds in the upper soil layers (Jacobsohn et al., 1980). This is effective only where incident sunshine is sufficient.

Host-Plant Resistance

Breeding resistant genotypes of host plants is one of the most promising approaches to reducing losses due to infestation by Orobanche (Cubero, 1991). Resistant sunflower cultivars were selected in Russia and Spain many years ago, and have been used successfully for decades. However, host-plant resistance was lost in many countries due to selection in the Orobanche populations towards more aggressive biotypes adapted to the newly introduced cultivars (Antonova, 1994). This could also be the cause for the very limited practical success of introducing genetic resistance against Orobanche in other crops, such as faba bean, lentil and tomato. Nevertheless, there are presently some resistant and some tolerant cultivars of faba bean (Cubero, 1994; Al-Menoufi, 1994), sunflower (Cubero, 1994) and vetch (Cubero, 1991), that can be used in infested fields after testing their fitness against the local Orobanche populations.

Biological Control

The broomrape-fly Phytomyza orobanchia was widely used for Orobanche control in the Soviet Union and some East European countries (Girling et al., 1979). No outstanding success has so far been achieved using this biological agent, mainly because of some hyperparasites (e.g. Opius occulisus) that attack Phytomyza.

Orobanche-specific Fusarium oxysporum was used to control the parasite in tobacco and in sunflower (Parker and Riches, 1993). This agent is still to be developed for worldwide biological control of the different Orobanche species.

Holm et al. (1979) list over 20 Orobanche species and indicate that one or other species is a major weed in Afghanistan, Arabia, Iran, Jordan, Egypt, Nepal, Tunisia, Hungary and Lebanon, and a 'principal' weed in Italy, India, Pakistan, Cuba and Turkey. Orobanche spp. are some of the most devastating pests in agricultural fields, mainly in the Mediterranean climate, with a tremendous economic impact. Orobanche crenata devastates the legumes that serve as an important source of protein in middle-eastern societies and other species cause heavy damage to vegetables and fodder crops in Mediterranean areas (Musselman, 1980; Foy et al., 1989; Parker and Riches, 1993). Sauerborn (1991) estimated that over one million hectares of faba bean in the Mediterranean region and western Asia are infested or at risk from O. crenata. In Morocco, the average level of loss in severe cases is 60% (Zemrag, 1994). Sunflower, an important source of oil in the world, is in danger due to severe infestation by O. cumana in several million hectares throughout eastern and southern Europe and in China. Heavily infested fields do not give any yield at all. According to Zhao and Yan (1983), O. coerulescens causes 38-70% loss in sunflower yield and 3-18% loss in oil production in West Jilin, China, and sunflower cannot be grown in some counties. However, Wang et al. (1990) do not list sunflower as a host of this species, and there must be some suspicion that the species involved is O. cumana. In Andalucia, Spain, almost all fields of confectionery sunflower are infected by O. cumana, while fields of oil sunflower are infected to about 50% (Parker, 1994).Infestation of tomatoes by O. ramosa can be especially dangerous as it may prove uneconomic to continue growing this crop (and many other vegetables) for a very long period. Attempts to resume growing a susceptible crop on infested land within 5-7 years are liable to result in immediate re-infestation (Parker and Riches, 1993). These are only a few examples of damage caused by Orobanche. The areas infested by the seven important species of Orobanche are vast and ever-growing, especially in modern times when international borders are no barrier for the transfer of Orobanche seeds.