Breaking Striga's Stronghold
Striga is a parasitic weed which presents a particular challenge to farmers in the tropics. Because of its unique life cycle, striga is extremely difficult to control and so can cause severe crop damage despite farmers’ efforts. Farmers need to know as much as possible about the plant and the various cultural techniques to help reduce the effects of Striga.
As a parasite, Striga is entirely dependent on its host-plants. Two of the most damaging Striga varieties, Striga hermonthica and Striga asiatica, parasitize such staples as millet, sorghum and maize. Another variety, Striga gesnerioides, attacks cowpeas and other legumes. An exudate from the roots of the crop plants stimulates germination of Striga seeds. The sprouting seeds attach themselves to the roots of the host crop (see inset) and draw off nutrients for their own growth. Striga undergoes considerable development below ground at the expense of its host. By the time the Striga plants appear at the soil surface they may already have devastated a crop. Three to four weeks after emerging, Striga plants flower. Each plant can producing up to 90,000 tiny, easily dispersed seeds which remain viable in the soil for up to 20 years.
Striga is most prevalent where rainy seasons are short, little rain falls, soils are of low fertility and where continuous cropping is practiced. Farmers in these areas are often at a loss to effectively combat this pest. Most of the control techniques available to them are too costly, are only partially effective or are effective only under certain conditions. Nonetheless, some farming practices can minimize crop losses by disrupting Striga’s life cycle. Striga’s total dependence on its host is the weak link in the Striga life cycle which provide the best opportunities for building control strategies. These strategies aim to prevent Striga from infecting host crops and to decrease the reservoir of Striga seeds in the soil.
Continuous cropping of a susceptible host crop favors the multiplication of Striga since the Striga plants are never without a host. Thus, one way to control Striga would be to rotate Striga resistant crops, such as peanuts, with Striga-host crops. Some farmers with enough land can grow the Striga-susceptible crop on a small part of their total cultivated land area, and move that crop to other areas in successive seasons. For many farmers, however, limited land resources do not permit them to leave any site free of the host crop long enough to control Striga, since the susceptible crop is also a vital staple. For farmers faced with limited land resources, intercropping Striga resistant crops with the susceptible ones is a viable way to assist in controlling Striga. Care must be taken in intercropping systems to relocate the Striga-susceptible crops each season, or the Striga infestation will continue at economically damaging levels.
“Suicidal germination” occurs when Striga is induced to germinate but dies for lack of a host-plant. Striga control strategies can take advantage of this principle to reduce the reservoir of seeds in the soil. Some plants are “false hosts"-- they can induce striga germination but are not infected by the parasite. in a trap-crop system, a false host such as sunflowers or castor beans is planted in rotation with the host crop. the false host induces suicidal germination, eliminating a large number of the weed’s seeds and reducing the severity of the following season’s striga infestation. this technique is limited to farmers who can afford not to plant the principle host-crop onthat site for a season.
in a catch-crop system, a susceptible crop is established at high density at the beginning of the rainy season to induce striga germination. after five to six weeks the crop is plowed in as a green manure and the striga is killed before reaching maturity. the main crop can then be planted. this method is only possible where a long rainy season permits the growth of two successive crops and where farmers have the equipment for incorporating the catch-crop and the weed in to the soil.
hand weeding of striga is considered to be a traditional control method. where the striga infestation is mild, this technique can be effective in preventing the weed from setting seed, but by the time the weed emerges and hand-pulling is possible significant crop damage has already occurred. in cases of severe striga infestation and where holdings are of more than a couple hectares, hand-pulling is far too labor-intensive for most farmers. hand-removal of newly emerged striga usually results in re-emergence since the tender shoots break off without detaching from the root of the host. if farmers wait until the stem is harder, weeding is more effective, but by that time the parasite has had more time to weaken the host.
striga is best adapted to conditions of low soil fertility. thus, improving soil fertility should aid in controlling striga. according to j. e. a. ogborn1, host plant tissue nitrogen is the critical factor to its resistance to striga. adding chemical or organic fertilizers to the soil can control striga, but not in all conditions. in extremely poor soils a small increase in fertility increases host vigor without increasing its tissue nitrogen. the slight benefit to the host crop also benefits striga.
appropriate amounts of organic fertilizers such as well-decomposed manure, on the other hand ha been usedagainst striga successfully. farmers are often unable to produce sufficient quantities of organic fertilizer in the same areas where striga is prevalent,. care must be taken that the manure is well-decomposed. manure that is not well-decomposed is a source of phosphorus, which improves the host (and parasite) vigor without increasing tissue nitrogen.
resistant crop varieties
one of the most promising approaches to controlling striga is the use of resistant crop varieties. various international research centers are working to develop resistant varieties of sorghum, maize, millet and cowpeas. the international crop research institute for the semi-arid tropics (icrisat) and the international institute of tropical agriculture (iita) are showing promising results. iita has discovered cowpea varieties which are completely resistant. they are also working with tolerant maize varieties that yield reasonably well even when infected. researchers at icrisat are working with sorghum varieties that produce low amounts of striga germination stimulant. they are also working to incorporate resistance from sem-wild millet into better agronomic varieties.
all the known methods of striga control have shortcomings. however, the sampling of control methods presented here shows that the weed is not invincible. more information is needed on ways in which farmers and researchers are successfully controlling striga using means available to small holders. those who have experience in this area are invited to share it with other readers of international ag-sieve.
ogborn, j.e.a., methods of controlling striga hermontheca for west african farmers, in samaru agricultural newsletter, vol. 12, no. 6, dec 1970.
striga identification and control handbook, information bulletin no. 15, by k.v. ramaiah, c.parker, m.j. vasudeva rao, and l.j. musselman. published by international crops research institute for the semi-arid tropics, 1983.
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