Genotyping of Cucumber mosaic virus isolates in western New York State during epidemic years: Characterization of an emergent plant virus population

• Partial sequences of 51 New York isolates of Cucumber mosaic virus obtained.
• Phylogenetic analyses identifies almost all isolates belong to subgroup IA.
• Legume associated codon Y631 of ORF2a found in viruses isolated from non-legume hosts.
• Infectious clones of representative snap bean infecting strain (Bn57) were produced.
• Symptom expression in bean is not dependent on a satellite RNA.
• RNA2 is the specific determinant for systemic infection in bean.
• Infection by transcript derived Bn57 causes significant yield losses in snap bean.

In the early 2000s an epidemic of cucumber mosaic virus (CMV) spread within the Midwestern and Eastern US affecting snap and dry bean (Phaseolus vulgaris L.) cultivation. Fifty one CMV isolates from this period were partially characterized from varied hosts by sequencing a section from each of the three genomic RNAs. Aside from one subgroup II strain from pepper, all isolates, including those from snap bean, fell within the IA subgroup. The nucleotide sequence diversity of virus populations sampled at multiple sites and at different years was significantly higher than that of a population from single site in a single year, although in general the number of polymorphisms was low (<11%). Complementary DNA (cDNA) clones of Bn57, a representative isolate from snap bean, were engineered for the production of infectious in vitro RNA transcripts initiated from a T7 promoter. Infections from these cDNAs resulted in symptoms consistent with those of the original field isolate, indicating that a satellite RNA is not involved in symptom expression in snap bean. These infectious clones were used to assess symptom determinants and the effects of virus infection on plant growth. Inoculations with pseudorecombinants derived from Bn57 and the non-bean infecting strain Fny confirmed RNA2 as a specific determinant for snap bean infection. Bn57, along with almost all isolates identified in this study contained the Y631 locus in the 2a protein, a determinant for systemic infection in bean. The presence of this locus extended to all non-bean hosts except two pepper infecting isolates. Infection by Bn57 in snap bean had a significant effect on pod number and mass with a 55 and 41 percent reduction in greenhouse assays, respectively. To our knowledge Bn57 is the first CMV strain isolated from P. vulgaris to be fully sequenced and cloned, providing a useful tool for analyses of CMV-host interactions.