Combine Controls to Manage Striga

Backgrounder

Striga

Striga, or witchweed, is one of the most important parasitic weeds in agricultural crops. It occurs in many parts of Africa, the Arabian peninsula, India, Indonesia and the United States. Local names are buda (Arabic), isona or rooibloem (Southern Africa), ndokoum (West Africa), bilikasa, talap, and urimallige (India). The economically most important species are, in decreasing order, S. Hermonthica, S. asiatica, S. gesnerioides, S. densiflora. Striga species cause yield reductions in sorghum, millet, maize, upland rice, and sugarcane. Also non gramineous crops are affected: cowpea, hyacinth or lablab bean, groundnut, and soybean. Direct yield losses may be as high as 60 90%, with additional costs of extra labour and other inputs!

Recognizing striga

Striga is a hairy, single stemmed or branched, (usually) annual herb that may grow from a few centimetres to over a meter. All species have opposite leaves and a reduced root sytem. The flowers are red, yellow, white, pink, or even blueish. Its parasitic mode of life is based on a root like structure (the haustorium), which brings about a vascular connection between the roots of the parasite and the host. Its size is in the range of 5 to 15 mm. Not all shoots of striga emerge, so the actual degree of infestation may remain invisible until most damage has already been done. The characteristic pattern is a very rapid emergence immediately after the end of the rainy season, slowing down to almost zero two or three weeks later. In rain fed crops the subsoil growth of striga usually coincides with the early growth stages of the host. The small seeds of striga (one kilogram may contain 270 million seeds!) are spread by the action of the wind, water or animals, the use of infested tools and contaminated seed.

Striga growth

Striga prefers light, sandy soils of lower fertility, a temperature range of about 18 to 40 degrees C, and low rainfall conditions. Under conditions of excess moisture Striga will fail to grow and develop. On poor, unfertilized soils the weed commonly strikes harder than on fields under high input management.

The seeds of Striga, which may remain dormant for up to 20 years, need a period of warm and dry conditions for after ripening. After a short period of moist conditions (the start of the rainy season, or the conditioning period), germination of the seeds is induced by a stimulus released from the roots of the host, usually the crop. About 5 to 8 weeks after germination (by the time the rainy season is over), striga starts to emerge above the ground. Experts do not know the ratio of aerial to subterranean (visible to invisible) striga, most probably because it differs from site to site. Flowering occurs about one month after emergence with the seeds maturing in 2 to 4 weeks. Flowering and fruiting may continue until the host plant dies; the full life cycle of striga usually takes 3 to 4 months.

The key to managing the weed striga, also known as witchweed (Striga spp.), is to use a variety of control methods at one time.

As many farmers well know, striga feeds on rice, maize, millet, sorghum, cowpeas, and sugarcane, and makes these crops sick. Striga can damage up to 70% of crops in a field. Striga grows from tiny seeds that can lie dormant in the soil for many years. The seeds germinate when they are near the roots of crops they like to feed on. The striga seeds produce root like structures which grow into the crop root and suck out water and nutrients. Affected crops are stunted and have wilted, yellow leaves. Striga grows mostly below the ground as it feeds on the crop. When it comes above ground it has already done most of the damage. Three or four weeks after it comes above ground, striga releases thousands of seeds. (See appendix.)

Rotate cereals with false host crops

Crop rotation is the first important control. It should be a priority. Avoid growing the same cereal or other main crop season after season. If you grow the same crop every year striga will continue to feed on that crop and become well established. By rotating cereal crops with other crops which are not affected by striga you make it hard for striga to survive. The best crops to rotate with striga are called “false hosts”. They include sunflowers, cotton, groundnuts, field peas, cowpeas, soybeans, sesame, lucerne, sunnhemp, pigeonpea, sesame, linseed, and castor beans. False host crops trick striga seeds into germinating but they are not infected by the weed because it cannot attach to their roots. Striga germinates but, because it cannot feed from the false host, it dies before it produces seeds. So there are fewer seeds for future striga growth.

If you do not have enough land to rotate your crops, try intercropping your cereals with false host crops or other crops that are resistant to striga.

Resistant cereal varieties

Another important part of striga management is to use resistant cereal varieties where seed is available. Talk to your extension agent about where you can get seed for striga resistant cereals.

Fertilize the soil

Use fertilizers that have lots of nitrogen, such as manure and compost. This keeps crops stronger and more resistant to attack. When your crop is stressed, as is often the case in low fertility soils, it is harder for it to resist striga attack. So fertilize your soil especially with nitrogen rich fertilizers.

Hand weeding

Hand weeding is an important part of controlling striga especially with small infestations. Pull out the striga weed as soon as it appears, before it flowers and reseeds. This will help control the spread of its seeds on your fields. Pull carefully to avoid breaking the striga stem. If the stem is broken the plant may spread even more. The plants should be completely uprooted, heaped, and burned when dry to prevent continued growth and seeding of the weed. Continue to hand pull throughout the season, and after harvest.