SUMMARY

In two distantly serologically related isolates (A and G12) of tomato black ring virus that each produce two RNA species, RNA-I was found only in bottom component (B) particles and RNA-2 only in middle component (M) particles. Preparations of separated M and B particles were each barely infective, but pro-duced 8 to 30 times more lesions when mixed, indicating that both kinds of particle are needed for infection, presumably because they contain different parts of the genome. Infectivity was not enhanced when M particles of isolate GI2 were mixed with B particles of isolate A, but it was increased when M particles of isolate A were mixed with B particles of isolate G12. The lesions thus produced were abnormally small; isolates cultured from some of them were slower than the parental isolates to produce systemic symptoms in Chenopodium quinoa and had serological properties indicating that their coat protein cistron is in RNA-2. These properties were stable on subculture. Pseudo-recombinant isolates were also produced when RNA-2 of isolate A was mixed with RNA-I of isolate GI2, but not with the converse combination