Outlook of cassava brown streak disease assessment: Perspectives of the screening methods of breeders and pathologists

Authors: OZIMATI, ALFRED - National Crops Resources Research Institute; ESUMA, WILLIAMS - National Crops Resources Research Institute; ALICAI, TITUS - National Crops Resources Research Institute; Jannink, Jean-Luc; EGESI, CHIEDOZIE - Cornell University; KAWUKI, ROBERT - National Crops Resources Research Institute

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/31/2021
Publication Date: 7/5/2021
Citation: Ozimati, A.A., Esuma, W., Alicai, T., Jannink, J., Egesi, C., Kawuki, R. 2021. Outlook of cassava brown streak disease assessment: Perspectives of the screening methods of breeders and pathologists. Frontiers in Plant Science. 12:648436. https://doi.org/10.3389/fpls.2021.648436.
DOI: https://doi.org/10.3389/fpls.2021.648436

Interpretive Summary: 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. The methods for screening CBSD resistance currently vary between breeders and pathologists, with the limited empirical data to support their choices. We used empirical CBSD foliar and root necrosis data from two breeding populations to assess and compare the effectiveness of the CBSD screening methods of breeders vs. pathologists. Estimates of broad-sense heritability (H2) using the breeder method ranged from 0.15 to 0.87, while for the pathologist method, they ranged from 0.00 to 0.71. Estimates of narrow-sense heritability using DNA markers (h2) on the breeder method ranged from 0.00 to 0.70 and from 0.00 to 0.63 on the pathologist method. In the second population, H2 ranged from 0.10 to 0.59 for the breeder method and from 0.09 to 0.35 for the pathologist method. In general, higher correlations between evaluations were found using the breeder method (r = 0.4, p = 0.01 at three months and r = 0.37, p = 0.01 at six months) than the pathologist method. Ranking of top 10 clones showed four overlapping clones between clonal and advanced selection stages for the breeder method while only a single clone overlapped with the pathologist method. Overall, the CBSD assessment method of breeders was more effective than the assessment method of pathologists, justifying its continued use in CBSD resistance breeding.

Technical 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.