The concentration of particles of BRNV, which is normally extremely low in herbaceous plants, increased c. 1000-fold when Nicotiana clevelandii plants were inoculated with a mixture of BRNV and the unrelated Solanum nodiflorum mottle virus. In sap from plants infected with the mixed culture, BRNV infectivity survived dilution to 10-4 but not 10-5, and storage for 6 but not 8 d at 20C, for 6 but usually not 10 d at 4C and >13 d at -15C. When plants were inoculated with the mixed culture, BRNV induced typical symptoms in several Chenopodium spp. and infected several previously unreported hosts. Purified preparations of particles of the mixed culture contained only a small proportion of BRNV particles, which sedimented in sucrose density gradients as 2 components, 1, probably non-infective, of c. 50S, and the other, infective, of 120-130S. An antiserum prepared to purified particles of the mixed culture was cross-absorbed with SNMV particles and used in indirect ELISA to detect BRNV in herbaceous plants infected with the mixed culture, and also in a wide range of Rubus spp., cultivars and hybrids infected naturally, by grafting or by inoculation with the aphid Amphorophora idaei. The reliability of ELISA for detecting BRNV in raspberry leaves depended on the cultivar and time of year. Glen Clova had low concn of BRNV, which was detected reliably only in late spring/early summer, whereas Lloyd George and Malling Enterprise had greater BRNV concentrations.
In small-scale surveys in eastern Scotland, BRNV was detected by ELISA in many raspberry cultivars, including some that contain major gene resistance to the vector, A. idaei; in 5 of 9 raspberry stocks entered for the Standard Grade certificate, but in none of 5 stocks entered for the Stock Cane certificate; and in 40% of wild raspberry and 14% of wild bramble plants growing near commercial raspberry crops. The significance of these findings for the control of BRNV is discussed.