Accumulation of yam viruses in different parts of white yam and the practice of positive selection technique in seed yam production

The success of Positive Selection Technique (PST) depends on the visual selection of virus-free plants, which may not always be possible due to virus accumulation in plant parts apart from the leaves. This study determined virus accumulations in different plant parts of white yam when practicing PST. Experimental plots were established at Ejura and Fumesua, and planted with planting materials (sourced from PST-practicing farmers) of three white yam varieties viz. Kpamyo, Dente and Pona. Symptomatic and asymptomatic plants from these fields were selected and yam mosaic virus (YMV), cucumber mosaic virus (CMV) and yam mild mosaic virus (YMMV) detected from their leaves, vines and tubers using DAS-ELISA. Virus concentration in the various plant organs were obtained from the absorbance values using BIO-RAD microplate reader. Disease incidence and severity were assessed at three months after planting. Data were subjected to ANOVA and means separated with LSD using the R software. In the subsequent season, harvested seed yams from symptomatic and asymptomatic plants were studied in the screenhouse and viruses detected in leaves and vines using RT-PCR. Plant viruses were found in all plant parts of the three white yam varieties. Concentration of all viruses in leaves, vines and tubers were significantly (p < 0.05) higher in symptomatic plants than in asymptomatic plants. Some seed yams which were selected as apparently “clean” planting materials could harbour yam viruses in plant parts, other than the leaf which is often targeted by PST.