Evaluation of Cassava Genotypes for Resistance to Cassava Mosaic Disease and Agronomic Traits

Sixteen cassava genotypes comprising introductions, local landraces and improved genotypes were evaluated for two seasons in Mansa, Zambia, for their reaction to cassava mosaic disease (CMD). The study was conducted in 2009/10 and 2010/11 seasons to evaluate the reaction of cassava cultivars to CMD and agronomic traits. Cassava mosaic disease severity and leaf retention were scored at 6 months after planting (MAP) and data on yield and yield components were recorded at harvest (7 MAP). Significant genotype x season interaction for CMD, harvest index, fresh root yield, biomass, plant height, root size and leaf retention was recorded. Bangweulu, Kalaba, Chikula, Mwakamoya and Chila-7 were the most susceptible genotypes over the two seasons. Mweru, Kampolombo, TMS190, TMS3001, Tanganyika and Nalumino had low severity scores. Harvest index ranged from 0.36 (Mwakamoya) to 0.55 (Chila-7) for the combined seasons. Chila-7 had the highest fresh root yield with a mean of 0.87 kg plant-1 for the combined seasons. The resistant genotypes might be used to improve the CMD resistance of local cultivars through hybridisation.

Genotype × environment interaction effects on early fresh storage root yield and related traits in cassava

Cassava(Manihot esculenta Crantz) is an important root crop worldwide. It exhibits substantial differential genotypic responses to varying environmental conditions, a phenomenon termed genotype × environment interaction(GEI). A significant GEI presents challenges in the selection of superior genotypes. The objective of this study was to examine the effect of genotype,environment and GEI on early fresh storage root yield(FSRY) and related traits in cassava.Accordingly, 12 cassava genotypes were evaluated in a randomised complete block design at three contrasting locations(Jinja, Nakasongola and Namulonge) in Uganda. Trials were harvested nine months after planting and the data collected were analysed using the additive main effects and multiplicative interaction(AMMI) model. The AMMI analysis of variance showed significant variation among genotypes for early FSRY and all other traits assessed.Locations were significantly different for all traits except for cassava brown streak disease root necrosis. The GEI effect was non-significant for early FSRY, but significant for other traits. For early FSRY, 48.5% of the treatment sum of squares was attributable to genotypes, 27.3% to environments, and 24.1% to GEI, indicating a predominance of genotypic variation for this trait.Predominance of genotypic variation was also observed for all the other traits. A majority of the genotypes(67%) had low interaction effects with locations for early FSRY, with Akena, CT2, CT4 and NASE14 being the most stable genotypes for the trait. Significant negative correlation was observed between cassava mosaic disease severity and early FSRY and storage root number,indicating significant negative effects of cassava mosaic disease on early FSRY and stability in cassava. The information generated will inform future selection initiatives for superior early-yielding cassava genotypes combining resistance to cassava mosaic and brown streak diseases in Uganda.

Interaction of genotype and environment effects on important traits of cassava(Manihot esculenta Crantz)

General and specific environmental adaptation of genotypes is the main goal of breeders.However, genotype-by-environment(G x E) interaction complicates the identification of genotypes for release. This study aimed at analyzing the effects of G x E interaction on the expression of important cassava traits using two multivariate analyses: additive main effects and multiplicative interaction(AMMI) and genotype stability index(GSI). Total carotene content(TCC), postharvest physiological deterioration(PPD), and reaction to viral diseases were significantly affected by G x E interaction effects. The low percent(%)variation due to genotype for cassava brown streak disease(GBSD) explained the influence of environment on CBSD expression. The % variation due to genotype for TCC was higher(96%) than variation due to environment(1.7%) and G x E interaction(2.4%) indicating a low interaction effect of environment on TCC accumulation. The % variation due to genotype was higher than % variation due to environment for all traits but CBSD root necrosis and CBSD on stems, indicating the influence of environment on the severity of the viral diseases. These findings indicate that screening for disease resistance requires multi-environment trials, whereas a single-environment trial suffices to screen for total carotene content.