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Striga, a food-crop pest : recent data on biology and control |
Cahiers Agricultures. Volume 2, Number 3, 167-82, Mai-Juin 1993, Synthèse
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 Résumé
Article gratuit
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Author(s) : Patrick Thalouarn, André Fer |
Summary : Among the parasitic weeds, Orobanche (broomrape) and Striga (witchweed) are extremely harmful to crops. They belong to two closely related families of dicotyledons : the Orobanchaceae and Scrophulariaceae. The genus Striga is widespread over intertropical Africa and south east Asia. Three species of the Striga genus (S. hermonthica, S. asiatica and S. aspera) cause almost all the damage observed in maize, sorghum, millet, rice and sugarcane fields, whereas S. gesnerioides is a pest of legumes, tobacco and sweet potato. Although the economic cost of witchweed is difficult to estimate accurately, it probably represents a loss of some billion US dollars for African farmers.
Syndromes and damage
Leaf scorching with necrotic spots on maize parasitized by S. hermonthica is the first symptom observed. Stunting and ear reduction occur with low-resistant varieties. When the crop is heavily infested no ears are formed. A syndrome rating scale used for measurement of maize tolerance to Striga is presented. Chlorosis is the first symptom of cowpea infestation by S. gesnerioides. Foliar dessication and reduction in number and weight of pods occur later. In the African savanna and Sahelian areas, losses in yield often exceed 15 % and in some cases may reach 80 or even 100 %. As a result, long fallow periods and even abandon of farms may occur. Poor soil, low precipitation, high density of inhabitants, and lack of crop-rotation are among the main factors increasing soil contamination by Striga seeds. In Nigeria, maize, sorghum and millet are grown on approximately 11 million hectares, 40 % of which are infested, mostly by S. hermonthica. The average loss in yield is estimated to be about 35 %, an economic cost of approximately ten billion French francs. In Africa, Striga is found in 40 countries and causes heavy damage in 17. Almost 40 million hectares are infested or threatened by witch weed.
Biology and physiology
Striga species are not only observed on crops during the rainy season. They have also been observed on several wild grasses. These alternative hosts have contributed in maintaining witchweed as an endemic pest in the African savanna zone. Roots are underdeveloped and fixed onto those of the host plant by means of a haustorium. In order to understand the harm caused by Striga, attention should be directed to its fecundity, which is reported to vary from around 10 000 to 100 000 seeds produced per Striga plant. Striga gesnerioides differs to some extent from other Striga. This species is nearly holoparasitic of wild legumes or cowpea by means of a large haustorium.
Seeds and germination
Striga seeds are very small (200 to 400 ?m in diameter) and, after ripening, remain dormant for several months. During the next rainy season, they become able to germinate after they have been conditioned in a warm, moist environment for several days. However, conditioned seeds must receive a chemical signal, provided by root exudates of host plants or non-host species (trap plants), for germination to be induced. These natural germination stimulants, as well as other less specific substances, seem to act by eliciting the synthesis of ethylene which could probably initiate the biochemical events leading to germination. The stimulants may also control the growth of the Striga seedling root towards the host root. Enzymes released by the growing Striga root liberate quinonoid compounds, such as 2,6-DMBQ which in turn trigger haustorial development.
Striga and water
In S. hermonthica and S. asiatica, rates of day-time transpiration are 4 to 8 times higher than in their host (sorghum). Transpiration rates in parasitized sorghum plants are 30 to 50 % lower than in unparasitized control plants. In these parasites, stomatal closure is low in response to environmental factors which usually control guard cell movements (darkness and water stress), even after treatment with abscisic acid. This anomalous behavior of stomata in iga may be a consequence of the high potassium content in the parasite’s leaves.
Striga always maintains its water potentiel to a value lower than that of its host. High transpiration rates and accumulation of inorganic and organic solutes in Striga are thought to maintain the water potential gradient between the host and the attached parasite.
Photosynthesis and carbon acquisition
Striga leaves are characterized by large numbers of stomata on both faces, a poorly differentiated palisade layer and a relatively low number of chloroplasts per cell. In S. hermonthica and S. asiatica, the value of net photosynthesis is about 5 fold lower than that measured in the host plant. Because of its low photosynthetic activity and high transpiration rate, Striga exhibits a very low water use efficiency.
These observations suggest that there is a requirement for host carbon to support the growth of the parasite. Indeed, Striga, which is entirely dependent upon its host during its subterranean stage of development, still receives appreciable additional amount of carbon from the host plant after emergence. Moreover, the parasite induces a strong reduction in its host’s photosynthetic activity \; this lowered photosynthetic rate may result largely from water stress conditions induced in the host.
Striga control
Propagation of Striga
The genus Striga has diversified widely since the beginning of the Third Era in Africa.
Two main factors may have contributed to the survival of the present Striga species : their fecundity (in terms of seed production) and their ability to parasitize wild plants. Over the past two or three decades, Striga infestation has spread dramatically due to erratic rainfall, less animal waste, the introduction of machinery and irrigation, increasing population and therefore intensive land use.
Striga asiatica was introduced accidentally into Carolina (USA) after the Second World War. The high rate of propagation caused the subsequent infestation of 200,000 hectares within a few years.
Breeding for Striga resistance in grain crops and cowpea Host plant resistance is known to be an inexpensive method of controlling Striga. Screening in pot cultures infested by S. gesnerioides has identified only one highly resistant line of Vigna unguiculata (B.301), a land race from Botswana. It was crossed to susceptible varieties which were already of agronomic interest and resistant to several insects and diseases. A single dominant gene « Rsg » was reported to be responsible for resistance to Striga. Although progress has been made in breeding for sorghum and maize varieties resistant to witchweed, no immune variety has been identified so far. The most successful breeding lines developed as now are known as resistant lines characterized by reduced losses in yield and a lower Striga seed production.
Resistance mechanisms
Among the almost 20 000 sorghum lines screened so far, approximately 4 % low-stimulant lines have been identified. These lines were tested in Striga - infested plots and 10 % were found to be resistant. Nevertheless, as some other resistant varieties exhibited a high production of germination stimulant such as sorgoleone, other resistance mechanisms are probably involved. One sorghum cultivar reported to be resistant has been found to induce haustoria at a great distance from the root, thus preventing attachment. It is likely that early formation of structural barriers in the host root tissues also accounts for the effective resistance of many varieties. Much work is needed to understand host resistance mechanisms and establish the degree of resistance of varieties reported to be resistant with any degree of certainty.
Chemical control of Striga
Various chemical control strategies have been developed in the USA for the eradication of S. asiatica over the last 30 years.
Depletion of seed reserves in cultivated soils can be achieved in two ways : inducing suicidal germination by ethylene or killing the seeds by fumigants such as methyl bromide. These methods are effective but they require specialized equipment and are very expensive. Control of Striga in non-crop areas can be achieved through the use of non-selective herbicides such as glyphosate and paraquat. Control of Striga in crops is much more difficult. Several soil-applied herbicides are able to reduce Striga emergence, but the herbicide used should be carefully selected in order to prevent damage to the crop species itself. Such herbicides also control non-parasitic weeds in the crop, some of which are host plants for Striga. However, soil-applied herbicides which control the parasite as it emerges or just before do not prevent the irreversible damage inflicted on the crop during Striga’s early subterranean development. Moreover, these herbicides do not result in a 100 % reduction of Striga emergence. Consequently, chemical control methods able to kill Striga after its emergence are also needed in order to prevent new seed production. Control of Striga after emergence can be achieved through the use of herbicides applied the foliage of the parasite.
Dicamba, 2,4-D, oxyfluorfen or linuron may be useful, provided that the herbicide is not damaging to the crop. Recently, it has been shown that a systemic herbicide such as Dicamba, applied as a spray to the foliage of sorghum or maize before emergence of Striga, provided good control of the parasite at its early stage of development without damaging the crops.
Existing chemical control methods need to be improved, perhaps using new more selective systemic herbicides, before they are used in Africa.
Other methods of controlling Striga
Many agronomists have suggested using zero or minimum tillage to avoid burying Striga seeds which could be brought to the surface in later years and result in reinfestation. Infestation by witchweed has always been associated with low soil fertility. It has often been reported that the application of either inorganic fertilizer or farmyard manure reduces both field infestation by Striga and losses in crop yields. When the degree of infestation reaches a high level, crop rotation should be used. This could be done with trap crops which induce the germination of Striga seeds but are not parasitized. Soyabean, cotton and groundnut are reported to be trap crops. Effective control through the use of long fallow periods is unlikely, as several African savanna grass species are alternative hosts to Striga. Intercropping is known to be an effective practice in reducing Striga emergence. The crops in the interrow may have one or several of the following effects : induction of abortive germination as a trap crop, increasing nitrogen in the soil, or decreasing the soil temperature which promutes inhibition of Striga seed germination.
Solarization is a method of soil disinfestation. By placing a clear plastic sheet over the soil, the resulting high soil temperatures reduce seed viability. However, the efficacy of solarization decreases with increasing soil depth, and more attention should be given to the viability of soil microorganisms. Since the control of parasitic weeds by the practices described above is difficult and often ineffective, hand pulling and hoe weeding are the most common practices used by small-scale farmers. A slight yield increase results from this tedious and labor-intensive operation but, overall, hand pulling could be envisaged as a long-term method for reducing the seed reservoir in fields. During the last two decades, biological control has not made decisive progress. Several insects feeding on Striga species are natural enemies with the potential to act as biological agents, and some efforts have been made to introduce them and achieve long-term control. But fungi, bacteria and even viruses could also be of interest in biological control \; this is a method that still needs much more work.
Conclusion
Alone, none of the methods described above can control Striga effectively. Progress can only be achieved through the integration of various methods in a package. Local conditions of the socio-economic environments should be taken into account to select the appropriate technologies. |
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