PATHOGENIC APTITUDES OF THREE Yam Mosaic Virus (YMV) GENETIC GROUPS RESPONSIBLE FOR YAMS VIRAL DISEASES OF THE COMPLEX Dioscorea cayenensis-rotundata IN TOGO

Abstract

This work was aimed to study the pathogenicity of genetic groups of yam mosaic virus (YMV) infecting Dioscorea cayenensis-rotundata in Togo. The virulence and aggressiveness of six YMV isolates (P3-20, C9-25, S2-21, M9-30, K9-5, P8-15) belonging to three genetic groups, were evaluated at the seedlings stage of three accessions of D. cayenensis-rotundata. These isolates were collected from the five yam-producing regions of Togo. For testing virulence and aggressiveness, two isolates per genetic group were mechanically inoculated to ten yam seedlings (five seedlings in two repetitions) per accession. Positive accessions were used to transmit YMV with the help of insect vector Aphis craccivora. RT-PCR test was performed to detect the presence of YMV in yam seedlings using two primers viz., YMV1 & YMV2 (196 bp). The results of the study revealed that all six YMV isolates were able to infect the three yam accessions. A significant difference (p = 0.05) was observed in the aggressiveness of the selected six isolates which clustered into three aggressiveness levels: group 1 included isolate P3-20 which was reported as very aggressive, group 2 have three isolates viz., S2-21, M9-30, K9-5 which considered as a moderately aggressive while group 3 have rest two isolates C9-25, P8-15 with a low level of aggressiveness. However, apart from isolates S2-21and M9-30 belonging to the same genetic and aggressiveness group, most of the isolates from the same genetic group had different levels of aggressiveness; suggesting that the variability in pathogenicity of these isolates was not, at first blush, related to the genetic variability of the virus. Also, it was reported that isolate S2-21 could not transfer through the vector A. craccivora. Results of the study suggested that among the tested YMV isolates, P3-20 was highly pathogenic while S2-21 and M9-30 were reported moderately pathogenic.