Alternative Approaches for Assessing Cassava Brown Streak Root Necrosis to Guide Resistance Breeding and Selection

Abstract

Cassava brown streak disease (CBSD) caused by the rapidly evolving cassava brown streak viruses (CBSVs), causes immense yield losses to the cassava value chain in eastern and southern Africa. Western Africa, another region that heavily depends on cassava is under eminent threat from CBSD. Resistance breeding is the best practical solution. However, complexities associated with CBSD resistance screening i.e., variable root sampling units, limit systematic attainment of genetic progress. Accordingly, we compared efficiency of five CBSD root necrosis assessment methods to guide selection: cassava brown streak disease root incidence (CBSDRi), cassava brown streak disease root severity (CBSDRs), cassava brown streak disease root severity computed as harmonic mean (CBSD-Harmonic), proportion-based root necrosis index (CBSD-proportion), and standardized root necrosis index (CBSD-standardized). The indexes (CBSD-proportion and CBSD-standardized) correct for variable sample size. We analyzed CBSD root necrosis data of 256 clones evaluated across 12 environments. Higher and variable standard errors were associated with root severity score 1 (no CBSD root necrosis). Lowest and highest plot-based heritability were respectively registered for CBSD-standardized (0.22) and CBSD-proportion (0.71). CBSDRs was only positively correlated with CBSDRi (r = 0.92) and CBSD-Harmonic (r = 0.97). Using best linear unbiased predictions (BLUPs), we ranked the top 15 CBSD resistant clones; only one clone (UG130014) featured in all the five assessment methods; two clones (UG130006 and UG120156) featured in four (CBSD-Harmonic, CBSDRi, CBSDRs, and CBSD-standardized); and five clones (UG120180, UG120063, UG130002, UG130033, and UG120183) featured in three methods (CBSD-Harmonic, CBSDRi, and CBSDRs). Influence of sample size was also quantified by sub-setting and analyzing CBSDRs data to have plots with at least 40 or 30 roots. Data stabilization was evident in plots with 30 roots. The significant influence of root sample sizes on overall ranking of clones, justifies the use of CBSD root necrosis indexes in early selection stages i.e., seedling and/or clonal trials, that are often characterized by high variations in roots assessed per plot. It is expected that this information will provide a foundation for harmonizing and/or optimizing on-going and future CBSD resistance breeding efforts.

Flour Pasting Properties Define Consumer Selection Decisions for Boiled and Flour-Based Cassava Food Products in Uganda

“…With each farmer growing three selected accessions, the best ranked accession for the farmer was assigned "score" three (3) while the worst ranked accession was assigned "score" one (1). Accessions ranked as medium in performance were thus assigned "score" two (2). The assigned scores were then used to select the best and worst performing accessions based on the ranking by farmers.…”

“…The variability observed for root softness in most varieties however pointed to the 23 possibility of using root softness as reliable metric to undertake selection in cassava [2]. 24 However, this should be coupled to explainable variations between varieties as was the 25 case in this study.…”

“…Cassava's (Manihot esculenta Crantz) ever-increasing role in food and industrial value chains justifies interventions aimed at sustaining and/or increasing its production and productivity [1]. It's partly for this reason that significant efforts and resources have for the past decades been devoted to genetically improve cassava and enable it overcome acute production stresses and/or obstacles to optimal production [2]. These breeding efforts led to increased adoption of new varieties i.e., adoption increasing from 20% in 1993 to 80% in 1999 and is still increasing up to now [3].…”

Outlook of Cassava Brown Streak Disease Assessment: Perspectives of the Screening Methods of Breeders and Pathologists

“…A discrepancy remains in the methodologies of CBSD resistance screening, which continues to be refined (Kawuki et al, 2019). In general, in screening for CBSD resistance, plant pathologists assess clone performance based on average foliar infected plants and/or roots, i.e., exclude scores of 1 FIGURE 1 | The broad-sense heritability estimates (H 2 ) for the three disease traits (cassava brown streak disease severity assessed at 3-months after planting [CBSD3s], cassava brown streak disease severity assessed at 6-months after planting [CBSD6s], cassava brown streak disease root severity assessed at 12-months after planting [CBSDRs]) for clonal evaluation trials (CETs-2) and advanced yield trials (AYTs) for the two mean CBSD computation methods.…”

Technological Innovations for Improving Cassava Production in Sub-Saharan Africa

“…Accurate evaluation and novel approaches for cassava disease detection are needed to efficiently assess disease severity ( Chiang et al, 2016 ). Root necrosis indexes for CBSD evaluation, which account for the root sample size or their economic value, have been proposed to replace traditional based evaluation ( Kawuki et al, 2019 ). Image analysis has similarly been used ( Garcia-Oliveira et al, 2020 ; Nakatumba-Nabende et al, 2020 ).…”