Use of positive selection technique to manage yam viruses in seed yam production by Ghanaian farmers

Positive selection technique (PST) involves physical examination of plants at vegetative stage for viral disease symptoms and using tubers from apparently symptomless plants as planting materials in seed yam production. This study determined PST use among farmers in Atebubu-Amantin and Ejura-Sekyedumase Municipalities of Ghana. Viral diseases were observed on 63 PST and 63 non-PST farms within these Municipalities. PST farms (two in third season and two starters) were selected to assess the impact of continuous PST practice on plant establishment, disease incidence, severity and yield. Harvested seed yams from asymptomatic and symptomatic plants were sampled for virus detections using Double Antibody Sandwich-Enzyme Linked Immunosorbent Assay (DAS-ELISA). Viral disease incidence (35.70%) and severity (1.51) observed on PST farms were significantly lower than non-PST farms with 54.11% and 2.80, respectively (p< 0.05). PST farms in the third season had higher plant establishment (66 plants/30 m2) and yield (48.32 kg/30 m2) with lower disease incidence (28%) and severity (1.50) compared to PST starters which respectively were 60 plants/30 m2, 33.14 kg/30 m2, 52% and 2.3 (p< 0.05). DAS-ELISA showed that, 85% of harvested seed yams from symptomatic plants were infected with YMV, CMV, or YMMV as against 35% seed yams from asymptomatic plants. YMV was highly accumulated in seed yams from symptomatic plants (0.344) than those from asymptomatic plants (0.269). The current findings show that PST if practised by farmers could minimise viral diseases in seed yams. However, poor observation of viral symptoms by farmers and latent viral infection of plants could be limitations which need to be considered in further studies. Key words: Double antibody sandwich-enzyme linked immunosorbent assay (DAS-ELISA), latent viral infection, virus accumulation, yam viruses.