- A high degree of reciprocal cross-protection occurred in sugar beets between beet soilborne mosaic virus (BSBMV) and beet necrotic yellow vein virus (BNYVV) in greenhouse experiments. The incidence of cross-protection depended on the interval between inoculations with protecting and challenging virus; longer inoculation intervals enhanced the incidence of cross-protection. Cross-protection was most effective when the inoculation interval was between 5 and 10 days, a period during which virus accumulated to a maximum level in plants singly infected with BSBMV or BNYVV. Results obtained by enzyme-linked immunosorbent assay (ELISA) and Western blotting were consistent and indicated that cross-protection affected viral capsid protein accumulation. RNA of both protecting and challenging viruses was detected in doubly infected plants by using reverse transcription-polymerase chain reaction (RT-PCR), indicating that RNA of the challenge virus was present in protected plants, but capsid protein was undetected by serological tests. BSBMV is a closely related but serologically distinct virus that provides an excellent system for studies of cross-protection with BNYVV.