Analysis of winter wheat and leafhoppers for wheat dwarf virus : factors causing outbreaks of wheat dwarf disease

Abstract: Wheat dwarf disease is caused by wheat dwarf virus (WDV) and is transmitted by the leafhopper Psammotettix alienus. The disease has been reported in many European countries, as well as in West Asia, China, and one country in North Africa. There are five strains (ABCDE) of WDV, with isolates of strains A, B and C infecting barley, and isolates of strains D and E infecting wheat. Although mixed infections can occur, in Sweden only the WDV-E strain has been found so far and the hypothesis is that it is still the same strain found in Sweden as the first sequenced WDV isolate from 1969. During the summer and autumn of 2022, symptomatic winter wheat and leafhopper samples were collected and distributed by the Swedish Board of Agriculture to investigate the presence of WDV from different locations. Winter wheat leaves from three counties (Södermanland, Västra Götaland and Östergötland) and six fields all tested positive for WDV with DAS-ELISA. Leafhopper samples from 28 locations distributed in the counties Uppsala, Stockholm, Södermanland, Örebro and Östergötland were tested for WDV through PCR with primer pair 1877-1896 and 328-309, which gives a PCR product of 1.2 kb. Three leafhopper samples, two from Uppsala (Haga, Säby) and one from Östergötland (Skarpenberga), tested positive and were further characterised with the help of cloning and sequence determination. Four clones from Haga and three clones from Säby were obtained and the sequenced isolates were analysed with the programmes BLASTx, BLASTn, MEGA11 and SDT. The sequences of the isolates from Haga and Säby showed 97.5% identity to those of other previously sequenced WDV-E isolates including the isolate from 1969. In a phylogenetic analysis, three sequences grouped together with subtype B also of the WDV-E strain. This subtype B has been found before in co-infections with the common genotype of WDV (Ramsell et al. 2008). In conclusion, the WDV isolates in Sweden have not changed much over time. It is still the same strain, with 97.5% nucleotide identity between isolates. Instead, main factors for outbreaks are probably volunteer plants, symptomless reservoirs, and dry climate with temperatures above 15 °C, which will increase the activity of the leafhopper.