Occurrence of Cowpea Stunt Disease Causing Viruses on Wild Bean in Arkansas

Cucumber mosaic cucumovirus (CMV) and blackeye cowpea mosaic potyvirus (BlCMV) interact synergistically in dually infected plants to cause cowpea stunt disease (1,2). During a July 1996 survey of cowpea stunt-affected fields in the Arkansas River valley, several wild bean (Strophostyles helvola L. Elliott) plants expressing mosaic symptoms were observed and collected. Sap was extracted from symptomatic leaves and used as inoculum to conduct a host range study. Virus symptoms diagnostic for BlCMV, CMV, or cowpea stunt were observed 6 to 8 days post-inoculation on Vigna unguiculata (L.) Walp. subsp. unguiculata cv. Coronet, while chlorotic lesions and mild systemic mosaic symptoms developed on Nicotiana benthamiana. Enzyme-linked immunosorbent assays (ELISAs) conducted on extracts from symptomatic cowpea and N. benthamiana 14 days after inoculation verified the presence of CMV and BlCMV in single and mixed infections. Additional symptomatic wild bean plants were collected from the same site in October 1996. ELISAs and Ouchterlony gel diffusion assays confirmed the presence of CMV and BlCMV, as well as soybean mosaic potyvirus (SMV), cowpea chlorotic mottle bromovirus, and alfalfa mosaic virus (AlMV) in symptomatic wild bean plants. While some of the collected plants were infected with only one virus, others had combined infections of CMV and BlCMV or these two viruses with AlMV. Interestingly, AlMV isolated from wild bean caused symptoms on greenhouse-grown cowpeas that were as severe or more severe than those caused by the cowpea stunt synergy, often resulting in plant death within 8 to 10 days after inoculation. The SMV from wild bean did not induce symptoms on cowpea and was not detected in extracts from inoculated cowpea plants by Ouchterlony gel diffusion assays, but was able to systemically infect soybean (Glycine max L.) cvs. Bragg and Lee. Another virus known to infect cowpea, southern bean mosaic sobemovirus, was not detected in any of the samples tested. To our knowledge, this is the first report that wild bean can serve as an alternate host for cowpea stunt-causing viruses in single and mixed infections. In addition, these data suggest that AlMV may pose a threat to Arkansas-grown cowpeas. References: (1) E. J. Anderson et al. Ark. Farm Res. 43:14, 1994. (2) G. Pio-Ribeiro et al. Phytopathology 68:1260, 1978.