Chapter Four

The growth stage of soybean (Glycine max (L.) Merrill) at the time of inoculation was determined to be a factor in the severity of seed coat mottling caused by Bean pod mottle virus (BPMV, Genus Comovirus, Family Comoviridae). The virus caused seed coat mottling, and reduced seed size and seed weight. The effects of different isolates of BPMV on seed coat mottling in soybean were found to vary depending on cultivar and stage of development of the plants at the time of inoculation. In 2002 and 2003, the effects of three and four BPMV isolates V-S98-1, V-S98-15 and V-S01-10, and V-W2, V-S98-1, V-S98-15 and V-S01-10, respectively, were investigated by inoculating four soybean cultivars, Bolivar, L8-379, Manokin and Williams. In both 2002 and 2003, plants were inoculated at three developmental stages; vegetative stages V1-V3 and V4V6, and reproductive stages R1-R3, in three replicates. In 2003, non-inoculated plants were included as controls. From the results of 2002, more obvious mottling was observed in early inoculated plants (V1-V3) than in plants inoculated at V4-V6 or R1-R3 stages of development. Generally, plants inoculated at R1-R3 produced less mottled seeds than those inoculated at earlier stages. Although the trends in seed coat mottling for early inoculated plants in 2003 were similar to those in 2002, some plants inoculated at R1-R3 in 2003, had higher incidence of mottling than those inoculated at the V1-V3 stage. Seed sizes also varied from V1-V3 to R1-R3 inoculation stages, with V1-V3 showing smaller seeds followed by V4-V6 and then R1-R3. The results show that the time of planting is a crucial decision if plants are to be protected from early inoculation by over-wintering leaf beetles. The time of planting can therefore be used as an important integrated pest or disease management component. A combination of judicious use of chemicals, correct planting dates and cultural practices like weed control, may substantially reduce early infection and improve quality as well as reduce virus-induced yield losses in soybean.