Studies of the distribution of yellowing viruses in the sugar beet root crop from 1981 to 1984

Two viruses, beet yellows virus (BYV) and beet mild yellowing virus (BMW), cause yellowing of sugar beet, their principal vector being Myzus persicae. Although the viruses have different properties which are likely to influence their spread within root crops, for the purpose of control they have been treated in the past as one disease. This paper describes the results of a crop survey carried out in co-operation with agricultural staff of British Sugar plc from 1981 to 1984 in which leaves from plants with symptoms of virus yellows were tested by enzyme-linked immunosorbent assay (ELISA) for BYV and BMW. The two viruses differed in their incidence and distribution within the national sugar beet root crop; BMW was the main cause of yellowing and occurred in all parts of the growing region while BYV, which has the more severe effect on yield, was more limited in distribution. The survey located areas in southern East Anglia which are at greatest risk from BYV infection. The possible need to modify control measures depending on which virus threatens to invade the crops is discusssed. A high proportion of leaves visually identified as infected was found to contain neither BYV nor BMYV, emphasizing the difficulties of identifying virus-infected plants by field symptoms.