The relation of annual incidence of beet yellowing viruses in sugar beet to variations in weather

Most economically important viruses of agricultural and horticultural crops in Great Britain ae aphid-transmitted and thei prevalence in any year depends largley on the numbers and earliness of aphid migrants during spring and summer. Aphids that fly very early in the spring are usually from colonies that have overwintered as adults on herbacewous hosts. Later migrants may be from eggs overwintered on trees or shrubs. Those that fly during the summer come from infestations that have built-up on the crops. All are influenced, directly or indirectly by meterlogical conditions during winter and early spring. Hurst (1965) studied the relation between various meterological elements and the prevalence in sugar beet crops of beet yellowing viruses transmitted by Myzus persicae Sulz. As in the present work, two viruses, beet yellows virus (BYV) and beet mild yellowing virus (BMYV) were considered together as causes of 'beet yellows' because surveys had not discriminated between them. Hurst concluded that the mean daily air temeprature in February was the main controlling weather factor, but that its effect could be modified by temperature in March and April. Relationships were established for groups of percentage infections and did not give precise predictions for individual years. Also the least accurate were for avergae and above avergae temperatures for which accurate forecasts are most needed, because the effect of very cold winters is predicatable from common experience of beet yellows outbreaks in sugar beet crops.