Studies on the luteoviruses beet western yellows virus and subterranean clover red leaf virus

Abstract

Beet western yellows virus (designated in this study as BWYVNZ) was shown to occur in New Zealand and cause yellowing in beet (Beta vulgaris L.) and turnip (Brassica rapa L.) crops in Canterbury. Subterranean clover red leaf virus (SCRLV) transmitted by Aulacorthum solani (Kalt.) and implicated as a cause of yellowing in beet, could not be isolated from beet showing leaf yellowing. This virus was shown, however, to be capable of infecting beet in the glasshouse. The host range of BWYV-NZ (natural and experimental) consisted of at least 44 species in 16 plant families. Symptoms induced in hosts consisted mainly of plant stunting, marginal or interveinal yellowing or reddening, and thickening of infected leaves. Myzus persicae (Sulzer) was an efficient vector of BWYV-NZ, having an acquisition threshold period of 5 min, an inoculation threshold period of 3 min and a latent period of 12-30 h. This aphid retained the virus for at least 17 days. Aulacorthum solani,Acyrthosiphon pi sum (Harris), Macrosiphum euphorbiae (Thomas) and Lipaphis erysimi (Kalt.) were also vectors of BWYV-NZ. An experiment in aphid-proof cages showed that infection of turnip cv. Green Globe with BWYV-NZ decreased total dry weight by 24%. Plants were inoculated at the 2nd-4th leaf stage and harvested 3 months later. Aphid flight patterns and the spread and incidence of BWYV-NZ and SCRLV were examined, during 1981-82, using bait plants. Plants became infected with both viruses from September to December and from March to May. Infection with BWYV-NZ was 8 times as frequent as infection with SCRLV. Myzus persicae was the most numerous aphid on the bait plants. From November to December 1981, M. persicae was the most abundant aphid on beet. From March to June 1982, the main aphids on turnip were M. persicae and Brevicoryne brassicae (L.). In 1981-82, incidence of yellowing in beet and turnip crops, presumably attributable to BWYV-NZ, reached 50-100%. An isolate of SCRLV was purified from Pisum sativum L. and an isolate of BWYV-NZ was purified from Crambe abyssinica Hochst. Using cellulase to assist in virus extraction. Yields of 0.3-3.0 and 0.2- 1.5 mg/kg tissue were obtained forSCRLV and BWYV-NZ, respectively. Particles of both viruses were isometric of a diameter of 27 nm and a sedimentation coefficient of 114 S. Purified preparations were infectious when fed to aphids through Parafilm M membranes. Electrophoresis of disrupted virions in gradient polyacrylamide gels revealed a single polypeptide of each virus of a molecular weight of about 22 200 for SCRLV and 23 250 for BWYV-NZ. Serologically, SCRLV was shown to be very closely related to soybean dwarf virus (SDV) and less closely to BWYV-NZ, bean leaf roll (BLRV), legume yellows (LYV) , Indonesian soybean dwarf (ISDV), potato leaf roll (PLRV), tomato yellow top (TYTV) and BWYV (isolate RY-I-R3 from California). BWYV-NZ was shown to be closely related to BWYV (RY-I-R3) and less closely to BLRV, LYV, ISDV, SCRLV, SDV, PLRV and TYTV. SCRLV and BWYV-NZ were detected in infected plant tissue by enzyme-linked immunosorbent assay. Properties of SCRLV and BWYV-NZ are consistent with their being definitive members of the luteovirus group.