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.

African Cassava Whitefly,Bemisia tabaci,Resistance in African and South American Cassava Genotypes

The whitefly,Bemisia tabaci,is a major pest of cassava,particularly in Africa where it is responsible both for the transmission of plant viruses and,increasingly,for direct damage due to feeding by high populations.To date,there have been no practical solutions to combat this emerging problem,due to the inability of the subsistence farmers that grow cassava to afford expensive inputs such as insecticides.A programme of research was carried out linking institutes in Africa,the UK and South America,to identify possible resistance sources in cassava to the whitefly,Bemisia tabaci.The South American genotype MEcu 72 and several Ugandan cassava landraces including Ofumba Chai,Nabwire 1 and Mercury showed good levels of resistance to B.tabaci.Field and screen-house experiments showed that all of the improved,high-yielding cassava mosaic disease（CMD）resistant cassava genotypes assessed were highly susceptible to B.tabaci and supported high populations of all life stages.These data support the hypothesis that the continuing high populations of cassava B.tabaci in Uganda are due,in part,to the widespread adoption of CMD-resistant cassava varieties during the CMD pandemic.They also show that the whitefly,Aleurotrachelus socialis,resistance present in the South American cassava genotypes could have broader applicability in the Old World.