| dc.description.abstract | mosaic disease (CMD) and cassava brown streak disease (CBSD) are the most devastating diseases of cassava in eastern Africa. Development of cassava with dual resistance to CMD and CBSD is still a challenge. In an attempt to develop dual resistance to CBSD and CMD, a CMD resistant cassava cultivar TME 204 was transformed for CBSD resistance. Two transgenic events showed durable field resistance to CBSD. However, these transgenic events were reported to show high level of susceptibility to CMD. To recover the lost CMD resistance, the CMD susceptible transgenic events were crossed with cassava cultivars that were conventionally resistant to CMD. The purpose of this study was to contribute to the faster development of cassava with dual resistance to CMD and CBSD. A total of 2921 F1 progeny were planted in an augmented block design with two local checks. The F1 progeny were assessed for resistance to both CMD and CBSD and then tested for presence of the CBSD gene. Additionally, F1 progeny were genotyped using four KASP markers to evaluate their effectiveness in tracking CMD2 resistance. Chi square goodness of fit tests showed that five out of six families fitted the 1:1 segregation of CMD resistant to CMD susceptible. But family TME 14 x T04026 deviated modestly from the expected segregation ratio of 1:1. The F1 population segregated in the ratio of 1 :3 for (transgenic and resistant to CMD) to (non-transgenic or transgenic but susceptible to CMD). Chi square tests of independence showed significant association between CMD phenotypes and KASP markers S12_7926132 (X2 = 463.03, p ≤ 0.001), S12_7926163 (X2 = 462.30, p ≤ 0.001) and S14_4626854 (X2 = 149.56, p ≤ 0.001). Additionally, regression analyses indicated that the three markers S12_7926132 (R² = 0.447), S12_7926163 (R² = 0.446) and S14_4626854 (R² = 0.132) were significantly associated with the CMD phenotypic scores. The KASP markers explained less than 50% of CMD variability which indicated a possibility that there were other factors other than the CMD2 gene responsible for CMD resistance in this population. However, it is worth noting that at both population and family level performance, markers S12_7926132 and S12_7926163 explained a higher proportion of variability than marker S14_4626854. Thus, the two markers, S12_7926132 and S12_7926163 are more strongly linked to the CMD2 resistance gene and would be effective for routine marker assisted selection for CMD resistance. | en_US |
