Survey of aphid-transmitted viruses and associated RNA molecules in Swedish sugar beet crops

Sammanfattning

Sugar beet is a crop of great economic importance, contributing significantly to global sugar production. Several viruses that aphids spread across the sugar beet crop have been managed with the application of neonicotinoid insecticides. The Scandinavian sugar beet industry has seen a rise in the number of diseases transmitted by aphids due to the prohibition of the use of neonicotinoids in 2018. The aphid-transmitted disease "virus yellows" has emerged as a major threat to Sweden's sugar beet crop in recent years. As of now, three poleroviruses, namely beet mild yellowing virus (BMYV), beet chlorosis virus (BChV) and turnip yellows virus (TuYV), have been identified in Sweden as associated with the disease. Beet yellows virus has also been identified in Sweden and can cause virus yellows. The objectives of this thesis were to study the diversity of aphid–transmitted viruses that infect sugar beet in the southern region of Sweden and to test for presence of turnip yellows virus-associated RNA (TuYVaRNA) in sugar beet. Symptomatic sugar beet leaves were collected from seven locations in Skåne in 2023. The samples were subjected to a double antibody sandwich enzyme-linked immunosorbent assay showing that samples collected from four locations were infected with poleroviruses. In this thesis, RT-PCR with universal primers for the coat protein gene of poleroviruses confirmed infection with polerovirus in these four positive samples as well as in one negative sample. Sequencing of cloned RT-PCR products revealed that three samples were infected by BChV and two samples by BMYV. In a phylogenetic analysis, one BMYV isolate showed a close relationship to a previously sequenced BMYV isolate from Sweden, while the other BMYV isolate represented a new genotype. The three BChV isolates were different from the previously sequenced Swedish BChV isolates and were most closely related to an isolate from France. The results showed that the diversity of BMYV and BChV in Sweden is higher than previously thought. New virus genotypes can be transmitted by migrating aphids from one country to another. Mixed infection of viruses could lead to emerging new virus variants as well. Using newly designed primers, RT-PCR amplification was carried out for detection of TuYVaRNA, but the tests were negative suggesting that the the tested plants might not have been infected with TuYVaRNA.