Transmission and host range of Horsegram yellow mosaic virus (HgYMV) causing common bean (Phaseolus vulgaris L.) yellowing disease in Sri Lanka

The common bean (Phaseolus vulgaris L.) yellowing disease (BYD) is caused by the Horsegram yellow mosaic virus (HgYMV), which is a begomovirus transmitted by whiteflies (Bemisia tabaci Genn.). It is a serious threat to common bean cultivation in Sri Lanka. Despite the fact the disease has been affecting bean production for a long time, its etiology and virus vector relationships are not understood well yet. Therefore, the present work was carried out to identify major aspects of the etiology of the virus. Initially a survey was conducted to determine the prevalence of the virus in representative bean growing locations in Sri Lanka, followed by a host range study using 54 plant species. Characteristics of virus-vector relationships were determined by conducting a series of virus transmission tests under glass house conditions. The survey results revealed the incidence of BYD at some time or the other in most of the sampled localities, which are scattered around all four major districts where the common bean is grown. Through the host range study, it was demonstrated that Ageratum sp., Glycine max Merrill., Macrotyloma uniflorum (Lam.) Verdc., and Phaseolus lunatus L. are susceptible to HgYMV, indicating a narrow host  range. In addition to the symptomatology, the successful entry of the virus following inoculation was confirmed through PCR using degenerate primers. Relative transmission efficiency was found to be 96.7 %, which corresponds to 0.24 of the estimated probability of transmission through a single whitefly (p*). The virus could be transmitted through probing by a single viruliferous whitefly with an incidence of 36.7 % (p* = 0.37). The minimum acquisition access period and inoculation access period were 20 min in each case and the respective disease incidence values were 30.0 % (p* = 0.04) and 36.7 % (p* = 0.05). The whiteflies could retain the virus in an active state for up to 9 days.