1. Assessing the impact of biotic agents and control interventions on seed yam degeneration
B.O. Osundahunsi1,2, A. Beatrice3, 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 *
Introduction
Yam (Dioscorea spp.) is a vegetatively propagated crop grown for its edible tubers in West Africa. Farmers recycle saved tubers as seed yam for planting. Uninterrupted recycling of farmer saved seed over decades has resulted in perpetuation of tuber-borne pests and pathogens contributing to reduced tuber yield and quality, eventually leading to seed degeneration (cumulative loss in yield potential due to pest and disease infection during different phases of crop’s life). Poor yields and quality Seed degeneration renders yam production unprofitable for farmers.
Objectives
To determine the causes and effects of seed yam degeneration due to sequential planting of saved seed, and the influence of positive selection, seed sorting and seed treatment in controlling seed degeneration
Table 2. Effect of chemical treatment on tuber quality after storage
Cultivar
Germinate (%)
Emergence (days)
Treated
Untreated
Makakusa1
77.86
76.20
27.50
26.80
Hembakwase1
93.59
92.54
24.70
36.70
Aloshi1
86.15
73.45
25.80
31.50
Aloshi2
87.50
90.00
24.00
Makakusa2
94.74
76.19
23.00 A C B
Reuse of seed from control trials (no positive selection) resulted in increase in mean virus incidence from 55.3 to 87.5, but no change in symptom severity (Table 3). Positive selection implemented in the farmers’ fields also resulted in reduction in virus incidence (mean ~30%) (Fig. 4). In this case farmers planted
Table 3. Summary of virus disease severity and incidence in trials without positive selection
Cultivar
Tuber quality rating at planting
Severity
Incidence (%)
2015
2016
Makakusa 1
2.3
56.0
87.4 2
2.4
2.6
67.4
96.8 3
2.5
47.6
91.0
Hembakwase
2.0
50.0
85.7
2.2
60.9
83.5
61.4
88.0
Aloshi
66.7
100.0
39.1
82.8
2.1
34.3
89.0 4
87.5
71.8
63.0
86.5
Mean -
55.3
Fig 1. Biotic causes of seed degeneration. (A) Virus infection, (B) Tuber rot due to fungal infection, and (C) Nematode infestation
Africa.
Materials and Methods
Trials were organized in IITA fields in Nigeria using D. rotunda (white yam) seed procured from local markets and virus-free seed generated at IITA. Seed yams were sorted based on a visual rating scale (1 to 5 severity score) and planted in 3 replications in an incomplete block design. Depending on the trial, seeds were treated with Macozeb and chloropyrifos. Rate of emergence, incidence and severity of virus, and fungal symptoms were recorded. Tuber weight and quality were assessed at harvest and after 2 months in storage. The tubers were replanted to assess the yield and quality loss between each generation. Positive selection (PS) was implemented by tagging asymptomatic plants (score 1, no symptoms) or mild symptoms in the lack of asymptomatic plants (score 2) in trial plots and also in farmers fields in FCT, Nigeria.
Results
Positive selection resulted in decrease in incidence and severity (Fig. 2). Seed treatment preserved quality of seed yam in storage (Table 1 and Fig. 3). However, seed treatment has nil to marginal effect on germination (Table 2).
Table 1. Effect of seed treatment on tuber damage during storage
Tuber severity score
Treated tubers (%)
Untreated tubers (%)
Pre storage
Post storage
Increase in damage
Score 1
81.5
78.6
2.9
84.1
66.5
17.6
Score 2
16.7
12.7 4
14.2
20.6
6.4
Score 3
0.6
7.2
6.6
1.5
7.9
Score 4
0.2
0.3
3.3
3.1
Score 5
1.7
Tubers harvested in trials was used for this trial
Fig 4. Incidence of virus symptomatic plants in the 50 farmer fields planted with PS seed (blue bars) and control farmers’ fields (no PS; control) (green line) in Virus incidence in the 50 farmers plots marked for PS in 2015 (red line).
Conclusions
Results of first season trials suggests positive selection, seed treatment and seed sorting techniques at different phases of the crop cycle have high potential to improve quality of seed yam and yield improvement. Further trials are necessary to evaluate effectiveness of various intervention in farmers’ fields to assess scalability of interventions under smallholder farming conditions.
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 2. Shift in proportion of plants by virus severity score due to positive selection
Fig. 3. Weight loss between treated and untreated tubers after 2 month storage.