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Researchers tame ‘purple witch’ with unique traps

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Concerted efforts between farmers and research institutions to keep the deadly striga weed at bay are finally paying off, after decades of disappointments and huge losses by farmers. Statistics indicate that
Striga, a highly invasive parasitic weed, infests 200,000 hectares of Kenya’s farmland and causes crop losses worth an estimated $50m each year by reducing yield by between 65 and 100 per cent.
Despite farmers’ maize fields having the same ecological characteristics and being only a metre apart, the yield from same-sized plots can sometimes vary by more than six bags because of the weed, which depletes once fertile plots.
The weed, also referred to as a witch weed due to its devastating effects on crops, attaches itself to the maize or sorghum roots from which it draws its moisture and nutrients, inhibiting plant growth, reducing yields and in extreme cases, causing plant death.

Unlike other weeds, which compete for water and nutrients, Striga, as a root parasite, literally sucks the life out of the crop on which it germinates. The invasion by the weed of maize, sorghum, millet and uplands rice has been so acute that farmers have sometimes even been forced to abandon their land.

“With increasing demographic pressure and demand for food, there has been intensification of land use, monocropping and consequently a decline in soil fertility. This depletion of soil fertility is one of the main causes for the increase in Striga incidence,” said Dr Lucas Alwayo from Maseno University’s Tegemeo Institute, an agriculture think tank.

Western Kenya farmers have been among the severely affected by the weed with research showing more than 80,000 hectares affected in the area. Pilot tests to curb the weed therefore started from this area, but have since been successfully replicated in other areas of the country.

Controlling Striga and other root parasites is made hard because the weed can do a great deal of damage to the host crop before emerging above the ground. But now a mix of cultural, mechanical, chemical and biological control measures are being advanced to regulate the parasite population. Few of these techniques provide complete Striga eradication alone.

Perhaps the simplest solution is crop rotation of infested land with non-susceptible crops or fallowing. Rotation with non-host crops interrupts further production of the Striga seed and leads to a decline in the seed population in the soil. The use of trap-and-catch crops that induce the germination of Striga but are not themselves parasitised is currently one of the best methods to control agricultural root parasites.

Catch crops stimulate a high percentage of the parasite seeds to germinate but are destroyed or harvested before the parasite can reproduce, for example as a thick planting of Sudan grass at 20-25 kg seed per hectare that is sown and either ploughed in or harvested for forage at 6-8 weeks before the Striga seeds.

Ua Kayongo—“kill Striga” in the local vernacular—another of the many attempts to weed out the striga was developed jointly by the Weizmann Institute of Science, the Kenya Agricultural Research Institute (KARI), BASF The Chemical Company, and CIMMYT, with funding from the Rockefeller Foundation and BASF. Researchers used a natural mutation and conventional breeding to create the maize variety.

It resists imazapyr, an active ingredient in the imidazolinone group of herbicides, so seed comes coated with a low dose of herbicide. The chemical kills Striga sprouts as they seek to attach to maize seedlings. The maize crop grows healthy and, over several years of use, fields are cleared of residual Striga seed.

Scientists from the International Centre for Insect Physiology and Ecology (Icipe) have also developed a structured guide on “push-pull technology” to help farmers simultaneously combat stem borer moths, striga weeds and poor soil fertility.

Icipe says small-scale farmers who contribute more than 80 per cent of the continent’s maize production often lack the money to buy synthetic pesticides. The synthetics are also said to be harmful to the environment and often ineffective.

The push-pull strategy uses a novel combination of forage plants which, when inter-cropped with cereals, act as both a trap and a repellent for stem borers and Striga. The two plants being used by Icipe are napier grass, which attracts the moths, and desmodium, which produces chemicals that repel stem borers.

Napier, planted as a border around the main crop, ‘pulls’ moths away from the cereal, while desmodium is planted among the rows of maize or sorghum to ‘push’ away the borers. The plants offer fringe benefits as well, according to Icipe.

The desmodium roots generate several substances, some of which inhibit the germination of striga, while others prevent the weed from attaching to the cereal plants’ roots. In addition, ‘push-pull’ has contributed to the augmentation of livestock production, especially on small farms where pressure on land is high, since both napier grass and desmodium provide quality fodder for livestock.

The centre says the push pull technology has been adopted by more than 40,000 smallholder farmers in East Africa since it was launched 17 years, resulting in improved maize yields of between one tonne per acre to 3.5 tonnes. It is aiming to roll out the technology to one million people by 2020.

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