Establishing inoculum threshold levels for Bean common mosaic virus strain blackeye cowpea mosaic infection in cowpea seed

Bean common mosaic virus strain blackeye cowpea mosaic (BCMV-BlCM) is an important seed-borne virus infecting cowpea and is transmitted both by seeds and aphids. Infected cowpea seeds can act as primary source of inoculum for disease epidemics. Four field experiments were conducted during 2003 - 2006 to assess the role of different amounts of seed-borne inoculum in the dissemination of BCMV-BlCM virus in cowpea under field conditions. The identity of BCMV-BlCM was confirmed by ELISA and IC-RT-PCR. Plants infected at an early growth stage appeared to serve as the primary source for subsequent virus spread by aphids. The mean disease incidence during four field experiments reached 88-93% in plots sown with 10% infected seed. The disease incidence in plots sown with 5% infected seed recorded 46-63% while for plants raised from 3 and 2% BCMV-BlCM seed infection, disease incidence reached 32-49% and 17-23%, respectively. Mean yield losses in terms of seed yield per plant from four field experiments were 74 and 54% for initial seed infection of 10 and 5%, respectively. Seed infection of 2% BCMV-BlCM incidence resulted in an average of 24% mean seed yield loss/plant-1. The infection appeared to decrease the seed yield in terms of number and size. The BCMV incidence in harvested seed ranged from 0.3 - 19% for the different levels of initial seed infection. The field experiments demonstrated that sowing > 1% BCMV-BlCM infected seed can lead to significant losses in grain yield, while the spread of BCMV-BlCM infection resulting from sowing 1% infected seed did not significantly decrease seed yield. The role of establishing damage or inoculum thresholds from BCMV-BlCM seed-borne infections is discussed in the present study