Epsilon Archive for Student Projects

Abstract

Sugar beet is a crop cultivated for production of sugar. Many diseases can cause yield loss of sugar beet, where some are caused by viruses. Virus yellows is caused by several viruses and is visible as yellowing of sugar beet leaves. These viruses are transmitted by aphids and can be found across the world in temperate regions. In Europe, the disease has long been known to be caused by beet yellows virus (BYV; genus Closterovirus) and beet mild yellowing virus (BMYV; genus Polerovirus), also in Sweden. This study mainly focused on poleroviruses since more recently two other poleroviruses, beet chlorosis virus (BChV) and turnip yellows virus (TuYV), have been found also to be associated with virus yellows. BMYV has been known in Sweden for a long time and can have significant effects on sugar beet production. BChV had not before been detected in Sweden and TuYV had not before been found in sugar beet in Sweden. Mixed virus infections can result in increased symptoms and enable recombination, which is important for virus evolution. Virus yellows has been managed by using neonicotinoids to limit the vectors, but this use is not allowed any longer by the EU. With less means of control, an interest in understanding the viruses causing virus yellows was raised. The aims of this study were to develop multiplex reverse transcription polymerase chain reaction (RT-PCR), including evaluating primers for detection and distinction of BChV and BMYV, and to study the diversity of poleroviruses found in Swedish sugar beet. Detecting and identifying these viruses is important to understand the disease and find methods for restricting infection and to know how the situation is in Sweden and what to expect for the future.
Field samples of sugar beet were collected in the southern part of Sweden and in Denmark. To detect closely related poleroviruses, ELISA was used and gave nine positive samples. Thereafter in RT-PCR with general polerovirus primers, seven out of them were positive. Variable regions of the P0 protein gene were targeted for distinction of BMYV and BChV with two published primer pairs for each virus. The tests showed that all seven samples had both BMYV and BChV. The identities of the viruses were confirmed through sequencing. Multiplex RT-PCR with one primer pair for each virus was used to detect BMYV and BChV simultaneously.
Detecting BMYV and BChV using the specific primer pairs worked, but not all primer pairs were able to detect the viruses, and some gave significant background amplification, being not as reliable as thought to be. Improvements are needed for the RT-PCR with specific primers and for the multiplex RT-PCR. Sequencing of the coat protein (CP) gene revealed that two samples also were infected by TuYV, as part of a triple infection with BMYV and BChV. One of the clones of the CP gene showed signs of recombination of BMYV and BChV, which proves ongoing increasing diversity. The sequences of the Swedish isolates were more similar to each other than to other isolates, and in general similar to other European isolates. In this study, BChV and TuYV were detected in sugar beet for the first time in Sweden. Since TuYV is abundant in rapeseed it has probably come from rapeseed to sugar beet. With more viruses infecting sugar beet in Sweden and in mixed infections, the sugar beet production in Sweden might get more severe problems with virus yellows than previously thought. More research would help in understanding and restricting virus infections.