Distribution and infectivity of yellowing viruses in field-grown sugar-beet plants.

Beet yellows virus (BYV) and beet mild yellowing virus (BMYV) are introduced to sugar‐beet root crops in spring by winged aphids, and are spread within the crops from these foci of infection. The rate of spread is likely to be influenced by the speed with which the initially inoculated plants become sources of infection. Studies reported in this paper showed that when field‐grown sugar beet were inoculated at the 2–4 leaf stage with BYV, the virus was detected in the youngest leaves after 1 wk, in all leaves after 2 wk and was readily acquired and transmitted by Myzus persicae from these infected leaves. In contrast BMYV multiplied and spread more slowly in plants inoculated at the 2–4 leaf stage, and was not acquired and transmitted by M. persicae very efficiently early in the season. When plants were inoculated at the 15–20 leaf stage there was less difference between the two viruses. The consequences of the observations in relation to aphid control are discussed.