65% loss in mean tuber weight ‘Positive selection’ effect on the incidence of Yam mosaic virus in yams B.O. Osundahunsi1,2, B. Aighewi3, T. Oviasuyi1, B. O. Odu2 and P. Lava Kumar1* 1International Institute of Tropical Agriculture (IITA), Oyo state, Nigeria 2Obafemi Awolowo University, Ile Ife, Osun State, Nigeria; 3IITA, Kubwa, PMB 82, Abuja, Nigeria E-mail: L.kumar@cgiar.org Introduction & Objective Yam mosaic virus (YMV, genus Potyvirus) causes “mosaic disease” which is responsible for loss of yam (Dioscorea spp.) production by at least 40%. YMV is endemic in West Africa and its incidence in white yam (D. rotundata) exceed 80%. Uninterrupted recycling of farmer saved seed has been the lead cause for very high YMV incidence in white yam, reduced tuber yield and quality, eventually leading to seed degeneration. Resistant sources to YMV are not available. Positive selection (PS) approach (Aighewi et al., 2017) was implemented to break the disease cycle in the farmers fields. Effect of PS on YMV incidence and severity was presented in this report. Fig 4. Summary of relative virus incidence trends in farmers’ fields compared with farmer plots using PS seeds Fig 1. YMV infected white yam (A) and health plant (B), and tuber harvested from YMV-infected and health plants (Photo source: L Kumar) Infected Healthy Kg Weight of 20 tubers Approach Fifty farmers trained to implement PS as part of the CAY Seed Project were selected for this study. In each farmer plot incidence and severity of YMV in control plots (no PS) and the plots planted with seed yams harvested from PS plants. Plants were assessed for YMV and symptom severity on 1 to 5 scale, where 1 = no symptoms and 5 = most severe symptoms. PS involve selection of seed tubers from plants with no symptoms [severity score 1 (S1)] or mild symptoms (S2). Plants with severe mosaic (>S3) avoided as seed sources. Fields were assessed in 2015 as baseline, and in 2016 and 2017 assessed PS plots and controls. Fig 5. Summary of virus severity in farmers’ fields compared with farmer plots using PS established with positively selected seeds. Benefit of reducing YMV incidence Yield loss due to mild symptoms (S2) was up to 15%, and severe symptoms (S3 to S5) was >40%. Reducing proportion of severely symptomatic plants using PS results in up to 30% yield gain (Fig 6). 65% loss in mean tuber weight Fig 2. Plants selected (A) and rejected (B) based on PS plan. (Photo source: B Aighewi) A B Fig 6. Tuber yield in plants with mild (S2) and severe symptoms (S3) compared to healthy plants of three D. rotundata cultivars. Results YMV incidence in the farmers fields in 2015 (baseline) was >85% with a mean symptom severity of 3. Adoption of PS resulted in reduction in severely symptomatic plants by about 50% in one season and increased proportion of asymptomatic (S1) and mildly symptomatic (S2) plants (Figs 3 and 4). Mean severity reduced by 0.4 (Fig 5). Conclusions This study demonstrated high potential of PS in reducing YMV incidence by up to 50% in one season. PS has limitation in reducing incidence beyond 40 to 50%, which needs infusion of virus-free planting material. PS contributed to increase in asymptomatic or mildly symptomatic plants and consequent raise in tuber yields. PS offers effective solution to augment health of farmer saved seed and yam yields. References Aighewi, B., Osei, K., Ennin, S.A., and Kumar, P.L. 2017. Practices to improve the quality of seed yam produced by smallholder farmers: a seed yam production guide. 28pp. [ISBN 978-978-8444-88-6] Acknowledgements This research work is part of the CAYSeed Project funded by CSIR-CRI, Ghana. First author is a PhD fellow in the CAYSeed Project. Fig 3. Shift in proportion of plants by virus severity score due to positive selection in three zones in FCT, Abuja, Nigeria. Proportion of each category of symptomatic plants were presented as percent means.