The prevalence and spatial distribution of viruses in natural populations of Brassica oleracea

We report a survey of four viruses (beet western yellows luteovirus (BWYV), cauliflower mosaic caulimovirus (CaMV), turnip mosaic potyvirus (TuMV), tu rnip yellow mosaic tymovirus (TYMV)) in five natural populations of Brassic a oleracea in Dorset (UK). All four viruses were common; 43% of plants were infected with BWYV, 60%, with CaMV, 4300 with TuMV and 1800 with TYMV. For each virus there were significant differences in the proportion of infecte d plants among populations, which were not completely explained by differen ces in the age of plants. Multiple virus infections were prevalent, with 54 % of plants having two or more virus types. There were statistically signif icant associations between pairs of viruses. The CaMV was positively associ ated with the other three viruses, and BWYV was also positively associated with TuMV. There was no detectable association between BWYV and TYMV, where as TuMV and TYMV were negatively associated. We suggest these associations result from BWYV, CaMV and TuMV having aphid vectors in common, as aphids a re attracted to plants that already hal-e a virus infection. Infected plant s were distributed randomly or were very weakly aggregated within populatio ns. The implications of widespread multiple virus infections in natural pla nt populations are discussed with respect to the release of transgenic plan ts expressing virus-derived genes.