RESISTANCE OF COWPEA AND CEREALS TO THE PARASITIC ANGIOSPERM STRIGA

Striga species are parasitic angiosperms that attack many crops grown by subsistence farmers in sub-Saharan Africa and India. Control of the parasite is difficult and genetically resistant crops are the most feasible and appropriate solution. In cowpea, complete resistance to Striga gesnerioides has been identified. Breeding for resistance in sorghum has identified varieties with good resistance to S. asiatica in Africa and India. One variety was also resistant to S. hermonthica in W. Africa. No such resistance to Striga has been found in maize or millets. Resistant varieties have usually been sought by screening germplasm in fields naturally infested with Striga. However, laboratory techniques have also been developed, including an in vitro growth system used to screen cowpeas for resistance to S. gesnerioides. Two new sources of resistance in cowpea have been identified using the system. The technique has also been used to investigate the mechanisms of resistance in this crop. Two mechanisms have been characterised, both were expressed after penetration of cowpea roots by the parasite. The resistance of some sorghum varieties to Striga is controlled by recessive genes. In cowpea, resistance to Striga is controlled by single dominant genes. The genes for resistance are currently being transferred to cowpea varieties which are high yielding or adapted to local agronomic conditions. One Striga resistant cowpea variety, Suvita-2, is already being grown widely by farmers in Mali. Reports of 'breakdown' of resistance in cowpea to Striga have not yet been confirmed, but a wider genetic base to the resistance is essential to ensure durability of Striga resistance.