An infectious full-length cDNA clone of the RNA genome of the potyvirus zucchini yellow mosaic virus (ZYMV) was constructed under the control of the cauliflower mosaic virus 35S promoter. All squash, cucumber, melon and watermelon plants inoculated with the cloned cDNA of ZYMV by particle bombardment become infected. Bombardment technology is 10(6)-fold more effective than mechanical inoculation. Due to the great increase in efficiency, ineffective constructs now became infective (i.e. cDNA under the control of the 35S promoter without the NOS terminator; with an addition of 127 nucleotides at the 5' end of the viral cDNA; uncapped transcripts), and the infectivity of capped-transcripts was maximized. Inoculation by particle bombardment produced visual symptoms rapidly (3-4 days), allowing the detection of viral coat protein and virions after 2 and 3 days in systemically infected leaves and inoculated cotyledons respectively.