Effect of cowpea seeds contamination rate by the Cowpea aphid borne mosaic virus on epidemics development.

Cowpea aphid borne mosaic virus (CABMV) diseased seeds provide at seedling, virus infected plants which are the only source of primary inoculum. Secondary infections are bequeathed by aphids. The objective of this research is to study the development of the secondary infection in field. Therefore, eight cowpea varieties with different seed contamination rate (0, 0.05, 0.25, 0.5, 1, 5%) were used over consecutive four years. The infected plants were recorded every week from the tenth day after sowing and over seven weeks. In the same way, aphids' population were evaluated in plots 30 days after sowing. There was no difference for the incidence rate between the average of plots sown with virus free-seeds and those sown with infected seeds with a rate of 0, 5%. In any case, the disease progressed lowly leading to incidences less than 50% at the post-flowering period in spite of a relatively high initial contamination rate of seed. For this group of varieties, the low progression of the disease indicated a high level of resistance to the infection. The high levels of infection especially observed with the varieties with high level of virus transmission to seed, translated the need to reduce aphids' population density notably by the use of insecticides during cowpea growing cycle. The high number of aphids and inoculum availability in the neighbouring plots were undoubtedly at the source of this result. This situation laid out the problematic of the use of seeds then little or not contaminated by the virus.