Two stains of cucumber mosaic virus (cucumovirus group), Fny-CMV and Sny-CMV, were used to genetically map phenotypic differences in severity and timing of symptoms in zucchini squash. Pseudorecombinants were constructed between the two strains, and the genetic purity of the pseudorecombinant strains was verified by molecular heterogeneity analysis of the viral RNAs using an RNase protection assay. Assessment of symptoms induced by the pseudorecombinant strains indicated that RNA 1 is responsible for both the severity and the rapid induction of symptoms elicited by Fny-CMV in zucchini squash. The potential role of RNA 1 in the induction of pathogenicity is discussed.