Occurrence and genetic diversity ofRaspberry leaf blotch virus(RLBV) infecting cultivated and wildRubusspecies in Finland

Abstract

Raspberry leaf blotch virus (RLBV) is a recently described (−)ssRNA virus (genus Emaravirus) associated with the long-known, severe raspberry leaf blotch disorder (RLBD). The virus is presumably transmitted by the raspberry leaf and bud mite (Phyllocoptes gracilis; Eriophyidae). Cultivated and wild raspberries (Rubus idaeus) displaying RLBD or yellowing symptoms were sampled in 14 districts in Finland and tested for RNA3 and RNA5 of RLBV by reverse transcription PCR (RT-PCR) and dot blot hybridization. A total of 59 samples were tested for RLBV, including 36 plants of cultivated raspberry, 20 wild raspberry plants, one ornamental purple flowering raspberry plant (Rubus odoratus), and two samples of P. gracilis (20 mites pooled per sample) collected from RLBD-affected Glen Ample. Fifty-three (93%) of the 57 plants tested were RLBV-positive, including seven raspberry cultivars (Balder, Glen Ample, Jenkka, Malling Minerva, Maurin Makea, Muskoka, Ottawa) and purple flowering raspberry, which is a new host for RLBV. RLBV was also detected by RT-PCR in mites (P. gracilis) collected from buds of RLBV-positive raspberry plants. The partial nucleotide (nt) sequence of the RLBV NP gene (nt 835–1284 of RNA3) was determined for 21 isolates obtained from 10 districts in Finland. Identical isolates were detected in distant districts, and distinctly different isolates were found in the same raspberry cultivation. Furthermore, eight different NP sequence variants were detected in the nine plants of a single raspberry cultivar (Glen Ample) tested from two districts. The nt and deduced amino acid sequences of the 21 isolates were 92.0–100% and 89.3–100% identical, respectively. Phylogenetic analysis revealed two main clusters. One cluster included three isolates, of which two isolates were from one farm in Finland and one isolate was previously characterised in Scotland. The other cluster contained the remaining 19 isolates characterised in this study. The results indicate that RLBV is widely distributed in cultivated and wild raspberries in Finland and shows considerable genetic variability.