DETECTION OF THE LUTEOVIRUSES, BEET MILD YELLOWING VIRUS AND BEET WESTERN YELLOWS VIRUS, IN APHIDS CAUGHT IN SUGAR-BEET AND OILSEED RAPE CROPS, 1990-1993

The incidence of beet mild yellowing luteovirus (BMYV) and non-beet-in fecting strains of beet western yellows luteovirus (BWYV) in individua l winged aphids, caught in yellow water-traps, in sugar beet during th e spring and early summer, and in oilseed rape plots in the autumn, wa s monitored using monoclonal antibodies in ELISA tests from 1990 to 19 93. Between 0% and 8% of the Myzus persicae trapped in sugar beet each year carried BMYV, whereas 0% to 4% caught in oilseed rape in the aut umn contained this virus. In 1990, 6.5% of Macrosiphum euphorbiae trap ped in sugar beet contained BMYV, but in subsequent years less than 1% were carrying virus. Much higher proportions (24-67%) of the M. persi cae tested from sugar beet contained BWYV, and similar proportions tes ted from oilseed rape (24-45%) also carried this virus in the autumn. In contrast only 3-19% of the M. euphorbiae caught in sugar beet conta ined BWYV, and none in oilseed rape. In 1991 and 1992 large numbers of Brevicoryne brassicae were caught in the plot of oilseed rape, of whi ch over 50% contained BWYV; none were carrying BMYV. In transmission s tudies between 1990 and 1992, 1% and 27% of M. persicae transmitted BM YV and BWYV respectively to indicator plants; subsequent ELISA tests o n the same aphids showed that 3% and 33% respectively contained the tw o viruses. One percent of M. euphorbiae transmitted BMYV, but none wer e found to contain BMYV using ELISA; 15% transmitted BWYV whilst only 5% were found to carry the virus. In 1992 and 1993 the incidence of BM YV-infection in the sugar-beet fields in which aphids had been trapped ranged from 1.2%, in a field which had received granular pesticide (a ldicarb) at drilling plus three foliar aphicidal sprays, to 39.5% in a field which had received only one foliar spray. In 1992 in a sugarbee t crop which had received no aphicidal treatments, and where 2.8% of i mmigrant M. persicae and 2.5% of M. euphorbiae contained BMYV, 11.6% o f plants developed BMYV infection. Lowest levels of infection were ass ociated with the use of granular pesticides at drilling. In 1990, 80% of oilseed rape plants in a field plot were infested with a mean of se ven wingless M. persicae per plant by mid-December; 37% of these plant s were infected with BWYV. The studies show that M. persicae is the pr incipal vector of BWYV, and large proportions of winged M. persicae ca rry the virus, in contrast to BMYV, which is consistent with the commo n occurrence of BWYV in brassica crops such as oilseed rape.