Strategies to stop Striga
Striga (Stringa hermonthica and Striga asiatica), also known as witchweed, is a parasitic weed that plagues cereal crops including maize, millets, sorghum and upland rice, in the semi-arid areas of sub-Saharan Africa. This weed may be beautiful but it is deadly; 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. In doing so, growth is stunted and yields are greatly reduced. The UN Food and Agriculture Organization estimates that the parasite causes annual losses in excess of US$7 billion, adversely affecting over 100 million African people in West Africa alone. In some areas, the problem of Striga is so bad that farmers have been forced to abandon their land. Over the years, finding ways of controlling Striga has been the aim of many research programmes, but success has been limited. With Striga affecting so many crops, research has taken a tactical turn to stop this weed from spreading.
Strike one: Prevention is better than cure
In an integrated Striga management programme in Nigeria by the International Institute of Tropical Agriculture (IITA), one important part of the strategy is to reduce Striga damage by limiting dispersal of seed, which can get transferred by livestock hooves, through transport of dung, and by planting crops contaminated with Striga seeds. However, this is easier said than done as each Striga plant can produce anything up to half a million seeds. Each one is virtually microscopic and can remain viable in the soil for up to 14 years. IITA stress that use of parasite-free planting material is essential combined with crop rotation and nitrogen-fixing cultivars, which help to improve soil fertility as well as reduce the density of Striga. Soybean, for instance, produces enough root exudant, the stimulant required to make Striga germinate, but is a non-host crop which stops the parasite from developing.
Sunnhemp, or Crotalaria Spp (Crotalaria juncea and Crotalaria ochroleuca) is another leguminous crop, which can be grown as a green manure crop and used as an alternative to fertiliser. Working with farmers and alongside Tanzanian scientists from Ilonga Agricultural Research Institute and Sokoine University of Agriculture, a collaborative project funded by the DFID Crop Protection Programme has tested the use of the green manure in trials in the south of Tanzania on the shores of Lake Nyasa. The results have shown that upland rice farmers, who have grown Crotalaria for a year, have managed to get much improved yields - equivalent to using 30 to 40 kg of urea per hectare - which has proved too expensive for farmers to use. The results have been especially encouraging, as rice yields in the region have plummeted over the past 20 to 30 years from 15 to as little as two bags per hectare. And, like soybean, Crotalaria helps to reduce the Striga seed bank in the soil.
Strike two: Resistant varieties
Not all varieties of cereals are affected by Striga. Some have an inherent resistance to the parasite by exhibiting tolerance to the weed - growing well despite its presence - or preventing the growth of the weed at an early stage. In recent years, Striga-resistant varieties of maize have been released in Nigeria by IITA. More recently, varieties of sorghum which have shown some resistance to Striga have been tested in three areas of Tanzania, as another aspect of the collaborative DFID-funded project to find ways of controlling Striga. Farmers in the Lake Zone of Tanzania, near the town of Mwanza and in Central Zone Tanzania near Dodoma, have been actively involved in testing the varieties using their own criteria to make the selections. "They are particularly interested in having varieties that mature early, that produce well on land that is of low soil fertility, and where Striga is a problem," explains Dr Charlie Riches of the Natural Resources Institute working with the project. "They also want sorghum that is palatable."
The varieties eventually chosen by the farmer groups were originally bred at Purdue University in the US. These two varieties, which have since been released by the National Programme, offer some resistance to Striga. The two varieties, now called Hakika (Kiswahili for 'certain' or 'sure' due to its reliability to produce a yield) and Wahi (meaning 'early' as it is an early maturing variety) both produce low levels of the root exudate that stimulates Striga and thus provides some level of resistance.
Strike three: Hi-tech alternatives
Perhaps the greatest concern with Striga is not just how many crop species it already affects but for its potential to widen its host-range. "Striga is evolving and that is frightening", says Dr Jonathan Gressel, Professor of Plant Sciences at The Weizmann Institute of Science in Israel. For example, Striga never used to affect teff in Ethiopia, but now it does. Genetic modification could offer an answer, he says. Herbicides can be used to kill parasitic weeds provided they do not also kill the crop. But sometimes this resistance is possible through plant breeding and does not require genetic manipulation.
The Mexico CGIAR research centre (known by its Spanish acronym CIMMYT) together with The Weizmann Institute of Science with funding from the Rockefeller Foundation, have developed a low-dose imazapyr (a systemic ALS-inhibiting herbicide) seed coating applied to its adapted IR-maize germplasm. Small quantities of imazapyr applied to the seed before planting prevents the phytotoxic effect of Striga on the maize, which occurs even before the emergence of the Striga from the soil.
"Farmers often like interplanting their maize for example with cowpea," says Dr Gressel. "And with the seed coating this is still possible because the herbicide dissipates in the root of the maize and is therefore no threat to the crop alongside it." The seeds are $4 to $5 more expensive per hectare than other hybrid maize seed, but the potential gain from using it is huge, says Dr Gressel. "By putting the herbicide on the seed, they don't need to spray the whole field. This uses 10 to 20 times less herbicide than spraying - just half a milligram per seed. In 50 field trials [in Kenya], the average difference was a three-fold yield increase."
Commercialisation of the seed is in hand - CIMMYT is undertaking trials elsewhere in sub-Saharan Africa, the Kenyan government is undertaking its own field trials, and BASF (manufacturers of the imazapyr herbicide) are also involved. Although the seed coating/IR-maize combination offers one option in the control of Striga, the threat of the weed's evolution means that we need a flow of new answers from research says Dr Gressel. "No solutions are forever."
Date published: January 2004
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