Wheat dwarf virus (WDV; family Geminiviridae, genus Mastrevirus) is a single-stranded DNA virus transmitted by the leafhopper Psammotettix alienus and it periodically causes severe damage to winter wheat in Sweden. WDV is also present in large parts of Europe. Two strains of WDV are known, the wheat strain and the barley strain. To get better understanding of the genetic diversity of WDV in Sweden and compare to the situation in the rest of the world, several isolates from wheat, triticale, wild grasses and the insect vector were collected and partially sequenced. All isolates collected in Sweden were shown to belong to the wheat strain of WDV. From Turkey and Hungary, two barley strain isolates were collected and complete genome sequences were determined. WDV infection in wild grasses was shown to occur only sporadically in Swedish grasslands, even in samples collected adjacent to heavily infected winter wheat fields. This indicates that wild grasses are not important as a primary source of WDV for the insect vector. Infected grasses might instead act as virus reservoirs, enabling WDV to prevail without winter wheat. The diversity of the Swedish wheat strain isolates and available international isolates was shown to be low. In phylogenetic analyses, no clear grouping could be seen according to geographical origin or host. The partial sequences of barley strain isolates grouped into three distinct clades: one Central-European clade with isolates from Germany, Hungary and Czech Republic, one clade with isolates from Spain and one clade with the Turkish isolates. For future studies on the host specificity determinants of the two WDV strains, infectious clones, transmissible by Agrobacterium tumefaciens, were constructed. The infectious clone of the barley strain was constructed from the Hungarian isolate WDV-Bar[HU]. The clone WDV-Bar[HU] was shown to infect barley, oat and rye. The biological activity of the barley infectious clone was further confirmed by insect transmission and typical WDV particles were visualised by electron microscopy. An infectious clone was also constructed for a Swedish wheat strain isolate and was confirmed to be able to infect wheat. PCR-based techniques were developed for rapid detection of WDV and Oat sterile dwarf virus in their respective insect vectors. The methods will be useful when trying to predict the risk of virus infection in cereal fields
