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

Robert Sezi Kawuki,Leah Nandudu,Williams Esuma,Alfred Ozimati,Ismail Siraj Kayondo,Marnin Wolfe

doi:10.3389/fpls.2019.01461

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.

Highlights

Cassava’s (Manihot esculenta Crantz) clonal nature, tolerance to marginal soils and ability to provide diverse food and non-food uses have elevated its importance in several countries within southern, eastern and western Africa
Cassava brown streak disease (CBSD) root necrosis data from Namulonge (NaCRRI) was associated with lower standard errors compared to data generated from the other two locations, Ngetta and Serere (Figure 1)
cassava brown streak virus (CBSV) present a unique situation such that upon infection, characteristic CBSD symptoms are manifested on leaves, stems and roots, with consistently poor correlations of disease severities and/or incidences amongst foliar and roots (Hillocks and Jennings, 2003; Kaweesi et al, 2014; Okul Valentor et al, 2018)

Summary

Introduction

Cassava’s (Manihot esculenta Crantz) clonal nature, tolerance to marginal soils and ability to provide diverse food and non-food uses have elevated its importance in several countries within southern, eastern and western Africa. On aggregate, these regions produce and consume more than 54% of the world’s cassava (FAOSTAT, 2019), essentially highlighting cassava’s significance in Africa. CBSD-affected roots have been reported to show reductions of 30% for amylose content, 50% for amylopectin content and 15% for total starch content (Nuwamanya et al, 2015) It is for these reasons that cassava breeding efforts in most southern and eastern African countries currently prioritize CBSD resistance as a major objective

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