GENETICS OF COWPEA APHID RESISTANCE

Abstract

Cowpea is one of the most important dietary staples in tropical Africa. The cowpea aphid (Aphis craccivora) is a major pest of cowpea that causes damage from the seedling to pod bearing stage. The use of resistant varieties appears to be the best option for farmers in the tropics owing to its low cost. It has been observed that cowpea aphid resistant lines developed earlier show differential effects on aphid population from different geographical areas. The legume Innovation lab project has put together eleven lines as sources of aphid resistance and susceptibility to be screened at selected locations in West Africa and California Riverside. In Ghana, the seedling screening at Savannah Agricultural Research Institute (SARI) Station at Manga showed that 5 lines, 58-77, IT9K556-6, KvX-295-2-124-99, SARC-1-57-2 and CB27 showed resistance to A. cracivora. A cross between IT9K556-6 and a known susceptible line (Apagbaala) from Ghana showed F2 progeny segregating into the ratio 3 Resistant: 1 susceptible when infested with cowpea aphids and F3 population segregating into 1 Resistant 2 segregating for resistance 1 susceptible confirming that the inheritance in IT9K556-6 was conferred by a single dominant gene. The resistant lines IT9K556-6 and KvX-295-2-124-99 were also crossed to a known resistant line in Ghana (SARC-1-57-2) to determine allelic relationship of the resistant genes. F2 generation of IT9K556-6 x SARC-1-57-2 segregated into 15:1 resistant/susceptible ratio, which was expected indicating that two different genes may be responsible for the expression of resistance. F2-3 population fitted into the ratio of 9:6:1 resistant/segregating/susceptible ratio. This ratio fits into dihybrid ratio for dominance at two loci. The cross between KvX-295-2-124-99 and SARC-1-57-2 showed all to be resistant both in the F2 and F2-3 populations indicating that it is the same gene causing resistance in both lines Data from this study on the genetics of aphid resistance in these lines will help in accelerating the breeding program in future, including pyramiding of the different resistant genes in cowpea genotypes.