Virus yellows in sugar beets in Sweden and Europe

Sammanfattning: This thesis describes virus yellows in sugar beets and converse both older and new knowledge about the viruses, vector and host relations and the control measures against the disease. The aim is to explore new potential control measures and to predict how the disease may affect agriculture in the future. Sugar beets are affected by various diseases, with virus yellows being one of the most important ones. Virus yellows in sugar beets is caused by beet yellows virus (BYV), beet mild yellowing virus (BMYV), beet chlorosis virus (BChV) and beet western yellows virus (BWYV). Infections cause visual symptoms like leaf chlorosis, brittleness, necrotic spots and decreased growth. Infections result in sugar yield loss by the reduction in biomass and sucrose concentration as well as by increasing juice impurity levels. The host ranges of the viruses causing virus yellows are narrow to large and mainly include species in the family Amaranthaceae. Weeds and crops, which are susceptible to virus yellows can act as infection sources and should be taken into consideration if the disease is occurring in sugar beet crops. The green peach aphid (Myzus persicae) and the black bean aphid (Aphis fabae) are the most important vectors for the disease. Virus yellows is spread by viruliferous aphids via feeding. Aphid populations are influenced by the occurrence of their primary host plants, natural enemies and the weather. Warm and dry weather usually results in more migrating aphids, which can get caught in the wind and drift over longer distances. When the incidence of virus yellows has been high in Sweden, the cause has been that viruliferous aphids have migrated from the south. The temperature does also influence the number of overwintering aphids in clamps or on host plants. In the past, the temperatures in Sweden have disfavoured overwintering of active aphids. However, there is a great risk that overwintering aphids become more common in the future due to climate change. Since the early 1900’s, virus yellows has caused concern to both farmers and sugar manufacturers, which has led to international corporations and extensive research on the issue. Throughout the 1900’s, the incidence of virus yellows has fluctuated between low and high levels. During the last decades, the incidence of virus yellows has been low because of the development of effective insecticides and virus yellows forecasting. Control measures against virus yellows consist of early sowing, limiting clamps and overwintering sugar beets as well as of insecticide usage. New control measures are now of importance since the ban of neonicotinoids in the EU in 2018, which limits the most used control measure against vectors of virus yellows. The lack of effective chemical control measures against the highly insecticide-resistant M. persicae suggests that virus yellows might become more frequently occurring in sugar beets in the future. Development or introduction of new insecticides for sugar beet production, development of resistant cultivars and using biological control are control measures that should be explored in the future.