LAUSR

Cassava (Manihot esculenta Crantz) brown streak disease (CBSD) is currently one of the major constraints to sustained cassava production in Malawi. Its economic impact is mainly manifested in the storage roots where it causes pitting, necrosis and constriction. CBSD can effectively be managed by using resistant varieties as well as early harvesting especially if the varieties are early bulking (high storage root yield). However, development of resistant and high storage root bulking varieties requires an understanding of gene action controlling inheritance of the two traits. Currently, there is very little information in Malawi regarding the inheritance pattern and relative importance of general (GCA) and specific combining ability (SCA) of these two traits. Therefore, a study was conducted to determine mode of gene action as well as importance of GCA and SCA in the genetic control of CBSD resistance and storage root yield. Information generated is essential in the selection of parents and breeding strategies for an effective breeding programme. Thirty-six crosses were generated using a 6 × 6 North Carolina Design II and later evaluated for CBSD resistance and storage root bulking at two locations using a triple square lattice design. Data for the progenies were analysed using REML in Genstat (17th ed.) while family (crosses) data were analysed using a PROC Mixed procedure performed in SAS® 9.3 Software. The GCAf × location, GCAm × location and SCA × location interaction effects were highly significant (P < 0.01) for CBSDS and CBSDI. Pooled analysis across two locations showed highly significant (P < 0.01) variance among GCA due to females (GCAf) and GCA due to males (GCAm) for FSRY. These GCAf, GCAm and female × male interaction (SCA) effects accounted for 19.8, 33.4 and 46.7% of the sum of squares for CBSDS, respectively. The results suggested that additive gene effects predominantly controlled CBSD resistance and high storage root bulking. Four parental genotypes (Silira, Mulola, Phoso and Mkondezi) were identified as the best general combiners for the CBSD, high storage root bulking and other traits. Thirteen progenies exhibiting CBSD resistance and high storage root bulking were identified and selected for advancement.