Assessment of Cassava Genotypes for Agronomic Performance and Resistance to Cassava Mosaic and Cassava Brown Streak Diseases in Western Kenya

Abstract

Cassava is an important food crop in sub-Saharan Africa. More than a third of the region’s potential cassava harvest is continuously lost to pest and diseases. The most important of these are the cassava viruses: cassava mosaic geminiviruses (CMGs) and cassava brown streak viruses (CBSVs). The objectives of the study were to: (1) evaluate 23 elite cassava genotypes for variation in CMD and CBSD resistance parameters at Alupe, Kakamega and Kibos in Western Kenya; and (2) assess the 23 elite cassava genotypes for variation in agronomic traits and correlate this to CMD and CBSD resistance parameters. The study used twenty-three (23) elite cassava genotypes that had shown promise in terms of their resistance to both CMD and CBSD, and had been officially released or were in the final stages of official release in the five cassava project (5CP) countries, namely Kenya, Malawi, Mozambique, Tanzania and Uganda. The genotypes were evaluated using balanced Alpha Lattice design, with three replicates at Alupe, Kakamega and Kibos for an extended cropping cycle between 2015/16 and 2016/17. Data was collected at 1 month after planting (1 MAP), 3 MAP, 6 MAP, 9 MAP and 12 MAP, and analyzed using generalized linear models using Genstat Version 15 software. Mean separation was carried out using least significant difference (LSD) at P ≤ 0.05 significance level. Whiteflies (Bemisia tabaci Genn.) abundance was significantly (P ≤ 0.05) influenced by genotype, environment (location), month after planting (MAP) time and interaction. Agronomic traits were highly influenced (P ≤ 0.05) by the location, and less (P ≥ 0.05) by genotype and interaction. All genotypes were considered sweet as cyanide content score was between 3.00 and 6.00 across the three locations. Mean DM yield (t ha-1) was 5.49, but 3.69, 4.65 and 8.14 at Alupe, Kakamega and Kibos respectively. Mean CMD and CBSD incidence was 0.60 and 0.84, respectively across the three locations. Similarly, mean CMD and CBSD severity was 1.09 and 1.13, respectively across locations. Mean CMD and CBSD incidence and severity was higher in Alupe, compared to Kakamega and Kibos. There was significant (P ≤ 0.05) negative association between CMD, CBSD incidence and severity with all agronomic parameters evaluated. Additive main effect and multiplicative interaction (AMMI) model detected highly significant (P ≤ 0.001) environmental effects in response to CMD and CBSD severity and incidence, biomass and fresh root yield. Highly significant variation (P ≤ 0.001) was detected by the AMMI model against genotype, environment and GEI in response to CMD incidence and severity. The AMMI analysis for all traits studied showed that more than 50% of the variation in GEI sum of squares (SS) was accounted for by integrated principal component analysis (IPCA1). IPCA2 were non-significant, indicating that they largely captured random error. AMMI stability values (ASV) were used to determine stable genotypes for dual disease resistance and agronomic performance at Alupe, Kakamega and Kibos. Combined (dual) resistance was, therefore confirmed for CMD and CBSD incidence and not severity. Therefore, best stable genotypes based on combined ASV and ranking in response for dual resistance to both CMD and CBSD incidence across the three locations were Colicanana, F10-30-R5, Orera, Tajirika and Kizimbani. Best stable genotypes in response to agronomic performance across the three locations were Nase-18, F10-30-R5, Nase-3, Eyope and Tajirika. Finally, based on combined ASVs and ranking, best stable genotypes in response to dual resistance for both CMD and CBSD incidence and agronomic performance across the three locations were KBH/2002/066, Kizimbani, Nase-18, F10-30-R5, Tajirika, CH05-203, Nase-3, Eyope and Orera. It is recommended that these stable elite cassava genotypes be further screened for wider adaptability and dual resistance to CBSD and CMD, including agronomic performance under farmer conditions in diverse farming systems, vector and disease pressures, for the possibility of future varietal release.