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An in vitro growth system was used to investigate the expression of resistance of cowpea [Vigna unguiculata (L.) WaIp.] varieties B301 and 58-57 to the angiosperm parasite Striga gesnerioides (Wilid.) Vatke. Variety Blackeye was included in experiments as a susceptible control. Seeds of S. gesnerioides germinated on the roots of all varieties. Two different mechanisms of resistance were expressed in B301 and 58-57 after penetration of host roots by parasite radicles. The first mechanism was expressed in both varieties; host tissue around invading Striga radicles became necrotic in association with the early death of the parasite and lack of tubercle formation. On variety 58- 57, the necrosis response was expressed only when the roots were inoculated with Striga from Burkina Faso but Striga from Mali successfully parasitized the roots of variety 58-57. The second mechanism was observed only on variety B301. Striga radicles infected cowpea roots, tubercles were formed but remained at 1-2mm in diameter with very limited stem growth. No further development of the parasite occurred in vitro or when infected cowpea were grown in soil or vermiculite. Similarities between the expression of resistance of cowpea to Striga and of plants to plant pathogenic fungi are discussed. [ABSTRACT FROM AUTHOR]