Incidence of Soybean dwarf virus and identification of potential vectors in Illinois

Soybean dwarf virus (SbDV), which causes an important disease of soybeans in Japan, is persistently transmitted by aphids and is endemic in forage legumes in the United States. To determine the incidence of SbDV in Illinois, we collected clovers and forage legumes in a total of 49 Illinois counties in 2001 and 2002 and tested them for the presence of SbDV by reverse-transcription- polymerase chain reaction. SbDV was detected in 43% of red clover (Trifolium pratense), 10% of white clover (T. repens), and 3% of yellow sweet clover (Melilotus officinalis) plant samples. The dwarfing strain (SbDV-D) was the predominant strain detected in Illinois. In 2000, Aphis glycines, an aphid species that colonizes soybeans, was reported for the first time in North America. To determine whether A. glycines or aphid species found colonizing clover were vectors of SbDV, transmission studies were conducted. Aphids of the species Nearctaphis bakeri reproducibly vectored SbDV among red clovers, and from red clover to soybean. A. glycines did not transmit SbDV; neither did two other clover-infesting aphid species, Acyrthosiphon pisum and Therioaphis trifolii.