Incidence, Spatial Patterns, and Associations Among Viruses in Snap Bean and Alfalfa in New York

Beginning in 2000, snap bean (Phaseolus vulgaris L.) crops exhibiting virus-like symptoms on the foliage (leaf distortion, yellow and green mottling, and mosaic) were increasing in frequency on a regional scale across the northern United States and Canada (10,20). These crops also produced fewer pods or pods that were either twisted or necrotic (hence unmarketable). Across the Midwest and northeastern United States in 2001, virus infection in some late-season processing snap bean fields resulted in total yield loss (20). High temperature stress in snap bean during reproductive development can cause floral abscission (33), resulting in reduced pod number. High temperatures also disrupt fertilization and ovule development, which then leads to deformed pods because of reduced seed set (36). However, the combination of virus-like symptoms, presence of soybean aphid (Aphis glycines Matsumura) at sometimes high numbers, and positive serological assays from affected fields strongly suggested that the observed symptoms and yield losses represented an emerging problem of virus epidemics, which had been less severe in past decades (37). The most prevalent viruses found in preliminary serological assays of snap bean samples from New York in 2001 were Alfalfa mosaic virus (AMV), Cucumber mosaic virus (CMV), and the Potyvirus genus (10). AMV, CMV, and members of the Potyvirus genus are transmitted by several species of aphids in a nonpersistent, stylet-borne manner (32). If aphids are primarily responsible for spreading these viruses into snap bean fields, a likely virus source is alfalfa (Medicago sativa L.). Alfalfa dominates the central and western New York agricultural landscape, where snap bean production is also concentrated, and is a reservoir of several viruses (46), some of which are transmissible to bean (4). Alfalfa fields typically remain in production for 4 to 6 years, sometimes longer, but virus incidence in forages tends to fluctuate over time and location, either increasing or decreasing (7,41). Studies on virus spread have demonstrated gradients in the incidences of aphid-transmitted viruses in legumes when grown near to virus-infected forages (13,17). Another study focusing on aphid movement showed that densities of the pea aphid (Acyrthosiphon pisum (Harris)) alatae in a lima bean field increased immediately after a nearby alfalfa field was harvested, suggesting that the aphid migrated from alfalfa to the lima beans (23). An increase in the densities of A. pisum alatae was observed in dry bean fields immediately after adjacent alfalfa fields were harvested (45). Therefore, proximity to alfalfa could significantly increase the risk of virus infection in snap bean. Our first objective was to survey commercial alfalfa and snap bean fields in New York for the incidences of the aphidtransmitted viruses AMV, CMV, and Bean yellow mosaic virus (BYMV). Alfalfa is a host to AMV (46) and BYMV (30). To our knowledge, commercial alfalfa fields in New York have not been surveyed previously for CMV; however, in a previous study, an isolate of CMV from alfalfa was infectious to bean (8). Aphid involvement in the transmission of viruses may be reflected by aggregated patterns of diseased plants (27). Therefore, our second objective was to quantify the spatial patterns of virus-infected plants in the same alfalfa and snap bean fields. The third objective was to determine whether virus incidence in snap beans depended on (i) the proximity of the snap bean field to alfalfa and (ii) planting time during the season. The time of arrival of aphid vectors can vary within a season; therefore, plantings made at different times in the season may be at different risks to virus infection (16). A fourth objective was to determine whether there were positive associations among the viruses in terms of their presence in infected plants.