dsRNA extracted from leaves was fractionated on cellulose columns, analysed by polyacrylamide gel electrophoresis and detected by staining the gels with silver. Such analysis readily detected two main virus specific dsRNA components in herbaceous plants infected with nepoviruses but detection of these species in Rubus plants was erratic. Interpretation of the analyses was also complicated by the erratic occurrence of minor dsRNA species and of low mol. wt components (less than 1.0 x 106). Low mol. wt components occurred in extracts from different plant species, from plants infected with unrelated viruses and from apparently healthy plants. Despite these difficulties, virus specific dsRNA was readily detected in raspberry infected with the type and the resistance breaking strains of raspberry bushy dwarf virus and with cucumber mosaic virus. Some extracts from raspberry, symptomlessly infected with black raspberry necrosis virus, contained traces of three dsRNA species with mol. wts (x 106) of 2.2, 1.9 and 0.7. Although a few extracts from raspberry plants symptomlessly infected with raspberry leaf mottle and raspberry leaf spot viruses also contained traces of a 2.2 x 106 mol. wt species most contained no detectable dsRNA of mol. wt greater than 1.0 x 106.
Extracts of blackcurrant plants of cv. Daniel's September carrying the infectious variegation agent, of field-grown cv. Baldwin with symptoms of yellows disease and of glasshouse-grown symptomless cv. Baldwin contained a dsRNA of mol. wt 2.9 x 106. Extracts of blackcurrant cvs and breeders' selections with symptoms of reversion disease contained a dsRNA of mol. wt 1.8 x 106 but so also did extracts from some symptomless selections. However, none of these dsRNA components were found in extracts of symptomless field-grown plants of cvs Ben Lomond and Ben More.
Field grown plants of Tyee strawberry showing severe June yellows symptoms contained traces of four dsRNA components of mol. wt (x 106) of 3.5, 2.8, 1.4 and 0.9.
The possible nature of the dsRNA species detected in Rubus, Ribes and Fragaria plants is being assessed.