Striga hermonthica
(witchweed)

S. hermonthica may have developed in north-east Africa (Musselman and Hepper, 1986), but appears to have reached most suitable ecologies in western and central Africa long ago, while its spread in Ethiopia has occurred mainly in the past 100 years; there are parts of that country and of Kenya with a suitable climate to which it has not yet spread. The reasons for non-occurrence in most of southern Africa are not explained and the possibility of further spread southwards should not be ignored. It occurs in the Arabian peninsula but has not yet reached any further east.

All Striga species are prohibited imports to USA and Israel.

S. hermonthica, in common with most other Striga species, is associated with low-fertility soils, especially those low in nitrogen. Unlike Striga asiatica it occurs not only on light, sandy soils but also on heavy clays and even on vertisols. It is also favoured by low soil moisture, and rarely occurs on irrigated soils, but can tolerate abundant moisture for short periods. It is a plant of African savanna, almost invariably associated with cereal cropping and relatively uncommon in natural vegetation.

The natural host range of S. hermonthica is normally limited to Gramineae [Poaceae], but weak attachment to groundnut, cowpea, lablab and soyabean was obtained in pots by Andrews (1946) and there have been unconfirmed reports of infestation of groundnut and sesamum fields in West Africa. Apart from the wild hosts listed above, S. hermonthica is occasionally observed on Dactyloctenium aegyptium, Panicum walense, Eleusine indica, Paspalum scrobiculatum and Pennisetum violaceum and on Cynodon, Cymbopogon, Ophiuros and Brachiaria spp. Individual biotypes may have a narrower host range than the species. In particular, there are forms which attack sorghum but not pearl millet and vice versa (see Parker and Riches, 1993).

The biology and ecology of S. hermonthica are described in detail by Parker and Riches (1993). In brief, S. hermonthica is an obligate hemi-parasite, with green foliage capable of photosynthesis and thus at least partially supporting its own growth once established, but with a minute seed which has inadequate reserves to establish without a host. The germination biology is comparable to that of Striga asiatica. Seeds require after-ripening, sometimes for several months after shedding, and then conditioning in the imbibed state for at least a few days before they will respond to a germination stimulant such as sorgolactone in the root exudate of a susceptible host crop such as sorghum, or a related lactone such as strigol, exuded by a non-host crop such as cotton. In the absence of a stimulant, prolonged imbibition of the seeds usually leads to some degree of 'secondary dormancy'.

Germination occurs within 24 h of exposure to stimulant. The seedling root grows to 4 or 5 mm only before dying in the absence of a host. On contact with a host root, however, elongation stops and sticky hairs develop, anchoring it to the root surface while the intrusive organ develops and penetrates the cortex and endodermis, to make connections with the host xylem. There are no clear connections with the phloem, and most water, minerals and elaborated sugars and amino acids are apparently obtained from the host xylem. Until the parasite emerges, it is totally dependent on the host for all these, while even after emergence and development of green foliage, its photosynthesis is relatively inefficient and it continues to depend on the host for most of its carbohydrate as well as nitrogen requirements (e.g. Press et al., 1987). Rapid transfer of materials from host to parasite depends on high transpiration in the parasite, which is helped by the almost permanently open stomata of the parasite and favoured by low humidity. Germination and growth are generally favoured by low soil nitrogen. High nitrogen, especially in ammonium form, may have direct toxic effects on Striga seedlings (e.g. Pieterse, 1991), but other effects may be more important, including the suppression of stimulant exudation from the host, and other changes in the physiology of the host-parasite relationship (Press and Cechin, 1994).

Relatively high temperatures, 30-35°C, are optimal for germination and for growth. Dry conditions of soil and air are most favourable, and S. hermonthica rarely occurs in irrigated cereals, though wet conditions can be tolerated for short periods. Neither soil type nor pH is critical, S. hermonthica occurring on almost all soil types from sandy acidic to alkaline clay soils, as in Sudan.

Unlike S. asiatica, S. hermonthica is an obligate out-crosser, depending on a range of insects for pollination (Musselman et al., 1983).

The main insect natural enemies of S. hermonthica have been catalogued by Greathead and Milner (1971), Greathead (1984), Markham (1985) and Bashir (1987). Among the commonest are the Smicronyx spp., gall-forming weevils widespread in West Africa, which may greatly reduce seed production. The commonest species are Smicronyx umbrinus and Smicronyx guineanus, while a new species, Smicronyx dorsomaculatus, has recently been described, though this occurs mainly as a stem gall on the related Striga gesnerioides (Anderson and Cox, 1997). These are apparently specific to Striga and closely related genera, but have not yet been successfully exploited for biocontrol. Also widespread are the less specific lepidopteran feeders Junonia orithya and Stenoptilodes taprobanes, often causing conspicuous damage to foliage and flowers, respectively, but with no potential as biocontrol agents.

Records of fungal attack on S. hermonthica have been reviewed by Greathead (1984) and Bashir (1987). A further range of pathogens has been reported from Ghana (Abbasher et al., 1995) and individual species of Fusarium have been identified in Sudan (Kirk, 1993; Abbasher et al., 1996) and Mali (Ciotola et al., 1995). A number of these organisms are under further investigation as possible mycoherbicides.

Infestation of a cereal crop by S. hermonthica may be apparent before emergence from the soil, by the chlorotic blotches on the crop foliage. Uprooting may confirm the presence of the haustoria and young parasite seedlings on the root. For detecting the seeds of S. hermonthica in crop seeds, Berner et al. (1994) used a technique involving sampling from the bottom of sacks, elutriation of samples in turbulent flowing water and collection of seeds and other particles on a 90 µm mesh sieve. Striga seeds are then separated from heavier particles by suspension in a solution of potassium carbonate of specific gravity 1.4 in a separating column. Sound seeds collect at the interface and are transferred to a 60 µm mesh for counting.

S. hermonthica is most often confused with Striga aspera, a common plant in West Africa, occurring on rice, maize and many wild grass hosts. S. aspera has slightly smaller flowers but is readily distinguished (in West Africa) by the bend in the corolla tube occurring two thirds to three quarters up, rather than about half way. Also the bracts in S. aspera are narrower, 1-2 mm only and without a fringe of hairs. S. aspera is rare in East Africa, but there it is less readily distinguished from the local S. hermonthica on the basis of the corolla tube, and the bract character is more important. Other Striga species have different coloured flowers and/or more ribs on the calyx tube (see Parker and Riches, 1993 for a key to the agriculturally important species). Others with 5 calyx ribs include Striga gesnerioides, a short, fleshy parasite of cowpea in West Africa, and Striga densiflora, a white-flowered parasite of cereals in South Asia. Striga asiatica, a white-, yellow- or red-flowered parasite of cereals in Africa and Asia, has 10-14 calyx ribs. Those with 15 calyx ribs include Striga angustifolia, Striga forbesii and Striga latericea.

Striga forbesii differs from S. hermonthica in its 15 calyx ribs, salmon-pink flowers and broader, coarsely toothed leaves. It is widespread but sporadic across Africa and Madagascar, sometimes attacking cereals and sugarcane, especially in Zimbabwe and Tanzania; also on wild hosts, especially Setaria and Echinochloa spp. Biology and ecology are generally similar to those of S. hermonthica, as are the damaging effects on crops, but germination factors may be different (Jackson and Parker, 1991).

Striga latericea is very similar to S. forbesii but is a perennial with deeper brick-red flowers and an underground stolon system, parasitic on a range of wild hosts in East Africa and on sugarcane in Ethiopia.

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.

Varietal Resistance/Tolerance

No completely immune cereal varieties have yet been developed, but many sorghum varieties show high levels of resistance, at least under local conditions. Selection and breeding programmes in India and Africa have led to the development and release of many lines with at least reduced susceptibility, and these may be valuable as components of an integrated control approach (e.g. Carsky et al., 1996; older literature reviewed by Parker and Riches, 1993). However, traditional varieties in Striga-infested areas often show relatively high tolerance, and these may yield well in spite of heavy infestation.

In maize there are no effectively resistant varieties, though some show partial resistance, such as var. Katumani in Kenya (Baltus et al., 1994), and some more tolerant lines have been developed by the International Institute for Tropical Agriculture (Kim and Adetimirin, 1997). Work is now in progress to transfer high-level resistance into maize from wild relatives including Zea diploperennis (e.g. Lane et al., 1997).

Little progress has been possible with the out-crossing pearl millet, but there are promising results from work in progress to select and develop rice varieties with resistance to Striga species (e.g. Riches et al., 1996).

Cultural Control

Characteristics/weaknesses in S. hermonthica which may be exploited in cultural control measures include the following.

- Dependence on a susceptible host for establishment. Crop rotation avoiding a susceptible cereal will prevent new seeding and allow decline of the soil seed bank. In some areas, there may be alternative cereals which are not attacked (e.g. sorghum in a millet-growing area, or vice versa). Among the non-cereal crops, many are known to exude germination stimulant, though they cannot be parasitized. These trap crops, such as cotton, groundnut, cowpea and soyabean, are especially beneficial in causing suicidal germination and accelerating a decline in the soil seed bank. But they need to be sown at a time when Striga germination is likely to be high, usually early in the rainy season, before the onset of any secondary dormancy. Catch crops are susceptible cereals which may be grown at the beginning of the season or in short rains prior to the main season, to stimulate germination of the Striga. However, they need to be destroyed before the weed can mature and set seed.

- Preference for low nitrogen. Additional nitrogen fertilizer usually reduces Striga incidence, though not always, especially when applied as a single dose (see Parker and Riches, 1993). However, improved soil fertility is a vital key to long-term control, whether by organic, inorganic or green manuring, rotation with legumes, or agroforestry techniques involving mulching.

- Preference for dry conditions. Irrigation is rarely an option, but moisture conservation techniques may be beneficial. Any means of raising humidity will reduce Striga transpiration and its ability to draw nutrition from the host. Hence leafy crop varieties, dense, uniform planting and mixed cropping (see e.g. Carsky et al., 1994) all tend to suppress the weed.

None of the methods described above will, alone, provide complete control, and without complete control there is the certainty that surviving plants will mature and replenish the soil seed bank. It is therefore essential that manual, mechanical or chemical methods are used to destroy surviving plants. Hand-pulling is the commonest traditional technique, though a late hoeing or ridging may also be effective.

Chemical Control

2,4-D may be used to kill emerged S. hermonthica or to prevent it from maturing and setting seed in sole-crop cereals, but not where mixed with legumes. Pre-emergence treatment with chlorsulfuron and other sulfonylurea herbicides has proved selective in sorghum and maize in some experiments (e.g. Babiker et al., 1996), but practical field use of these herbicides is likely to depend on combination with herbicide-tolerant crop varieties. For instance, Abayo et al. (1996) report the useful selectivity of imazapyr and chlorsulfuron applied into the planting holes with seed of maize genetically engineered for tolerance of acetolactate synthase-inhibiting herbicides. In a similar study, Berner et al. (1997) applied the herbicides imazaquin and nicolsulfuron to the maize seed before sowing. It is likely that glyphosate will become a useful tool for post-emergence control of S. hermonthica in glyphosate-tolerant maize. It is less likely, however, that herbicide-tolerant varieties of sorghum or pearl millet varieties will be developed.

Biological Control

The reduction in seed production from gall-forming Smicronyx spp. is often substantial (e.g. Kroschel et al., 1995; Traore et al., 1995), but there has been no successful development of a biological control programme based on these weevils. Attempts to introduce Smicronyx albovariegatus (and the moth Eulocastra argentisparsa) from India into Ethiopia apparently failed. Meanwhile, conclusions from a mathematical modelling project have suggested that Simicronyx spp. would in any case be unlikely to have a significant impact on Striga population dynamics (Smith et al., 1993).

There is now rather more interest in the use of soil pathogens as mycoherbicides, especially Fusarium spp., including Fusarium nygamai (Sauerborn et al., 1996), Fusarium oxysporum (Ciotola et al., 1995; Kroschel et al., 1996), Fusarium equiseti (Kirk, 1993), Fusarium semitectum (Abbasher et al., 1996) and Fusarium solani (Kroschel et al., 1996). However, further work is needed before a reliable economic treatment is developed.

Integrated Control

As virtually none of the treatments described above is likely to achieve complete control, integration of one or more is essential for any substantial reduction of the problem. Furthermore, such integrated treatments will almost certainly need to be repeated over a number of years for long-term control. Parker and Riches (1993) propose a range of programmes depending on the initial density of the problem, involving various combinations of rotation, varietal selection, soil fertility enhancement and mixed cropping, supplemented in all cases by hand-pulling, herbicide application or other techniques to prevent seeding.