Abstract
Cassava production and productivity in Eastern, Central, and Southern Africa are ravaged by cassava brown streak disease (CBSD), causing yield losses of up to 100% when susceptible varieties are grown. Efforts to develop CBSD-resistant clones are underway. However, the methods for screening CBSD resistance currently vary between breeders and pathologists, with the limited empirical data to support their choices. In this study, we used the empirical CBSD foliar and root necrosis data from two breeding populations, termed cycle zero (C0) and cycle one (C1), to assess and compare the effectiveness of the CBSD screening methods of breeders vs. pathologists. On the one hand, the estimates of broad-sense heritability (H2) for the CBSD root necrosis assessment of breeder ranged from 0.15 to 0.87, while for the assessment method of pathologists, H2 varied from 0.00 to 0.71 in C0 clones. On the other hand, the marker-based heritability estimates (h2) for C0 ranged from 0.00 to 0.70 for the assessment method of breeders and from 0.00 to 0.63 for the assessment method of pathologists. For cycle one (C1) population, where both foliar and root necrosis data were analyzed for clones assessed at clonal evaluation trials (CETs) and advanced yield trials (AYTs), H2 varied from 0.10 to 0.59 for the assessment method of breeders, while the H2 values ranged from 0.09 to 0.35 for the CBSD computation method of pathologists. In general, higher correlations were recorded for foliar severity from the assessment method of breeders (r = 0.4, p ≤ 0.01 for CBSD3s and r = 0.37, p ≤ 0.01 for CBSD6s) in C1 clones evaluated at both clonal and advanced breeding stages than from the approach of pathologists. Ranking of top 10 C1 clones by their indexed best linear unbiased predictors (BLUPs) for CBSD foliar and root necrosis showed four overlapping clones between clonal and advanced selection stages for the method of breeders; meanwhile, only a clone featured in both clonal and advanced selection stages from the CBSD assessment method of pathologists. Overall, the CBSD assessment method of breeders was more effective than the assessment method of pathologists, and thus, it justifies its continued use in CBSD resistance breeding.
Highlights
The human population in the 30 years is projected to increase by 25%, from the current world population of ∼7.5 billion to 10 billion people
The average broad-sense and narrow-sense heritability estimates across trials were higher for the cassava brown streak disease (CBSD) assessment method of breeders (H2 = 0.56 and h2 = 0.36) than for the CBSD assessment approach of pathologists (H2 = 0.49 and h2 = 0.25) (Table 1)
We observed higher H2 values for the CBSD assessment method of breeders compared with the CBSD assessment method of pathologists for both clonal evaluation trials (CETs) and advanced yield trials (AYTs) evaluation stages
Summary
The human population in the 30 years is projected to increase by 25%, from the current world population of ∼7.5 billion to 10 billion people. The average on-farm yield of cassava in Africa is low, stagnating at 12 tons/ha compared with 20 tons/ha estimated for Asian and Latin American countries (Malik et al, 2020). The biotic factors, such as cassava brown streak disease (CBSD), cassava mosaic disease (CMD), cassava bacterial blight, and whitefly vector, are the key obstacles to optimal cassava production and productivity in Africa (Maruthi et al, 2005; Mware et al, 2009; Patil and Fauquet, 2009; Patil et al, 2015). Typical cassava plants infected with CBSD present characteristic yellowing along the veins, compromising the photosynthetic capacity of leaves, brown streaks on stems, and corky necrosis in the edible root parenchyma, and rendering the roots unusable for food or feed (Hillocks, 2004; Patil et al, 2015; Hillocks et al, 2016)
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