Pathology, soil transmission and characterization of cymbidium ringspot, a virus from cymbidium orchids and white clover (Trifolium repens)

Abstract

SUMMARYTwo strains of a virus, designated cymbidium ringspot virus (CyRSV), were isolated from cymbidium orchids and from Trifolium repens respectively in Britain. Experimentally infected cymbidiums developed slight chlorotic ring‐mottle; T. repens developed flecks and mottling in the leaves, and slight stunting. Of 101 plant species tested, the cymbidium strain infected sixty‐one (thirteen systemically) in twenty‐three of thirty‐five families; the clover strain infected sixty‐four species (eighteen systemically) in twenty‐two families. Both strains were propagated in Nicotiana clevelandii and assayed in Chenopodium quinoa.CyRSV was readily transmitted by inoculation of sap, and by foliage contact between plants, but not by the aphids Myzus persicae or Acyrtho‐siphon pisum, nor through seed of T. incarnatum, Phaseolus vulgaris or N. clevelandii. Highly infective virus was released into soil from roots of infected N. clevelandii, and acquired by bait seedlings planted in such soil. Similar transmission occurred when purified virus was applied to the surface of sterilized soil containing bait plants; there was no evidence for any living soil vector.The virus was eliminated from 96 % of small cuttings taken from infected N. clevelandii plants grown at 35–37 °C for 9 wk.CyRSV was still infective in sap of N. clevelandii after dilution to 10˜5‐io–6 (only 2 × 10_1 in cymbidium sap), or after 10min at 85–90 °C. It survived at least 10 months at c. 20 °C and more than 12 yr at 2 °C. Lyophilized sap was highly infective after over 13 yr at laboratory temperatures under high vacuum.Purified preparations made by clarification with n‐butanol, followed by differential centrifugation and exclusion chromatography on controlled‐pore glass beads, contained isometric particles c. 30 nm diam., with s°20W= 137 S, and had a buoyant density in caesium chloride of 1–36 g/ml. The A 260/A 280 ratio was 1–55, and A max(26o)/A min(242) was 1–17. The virus contained c. 15 % of single‐stranded RNA of mol. wt 1–7 × 106; the nucleotide base ratios were: G27'8; A24/9; C2I‐3; U26‐I. There was one capsid polypeptide of mol. wt 43600.The virus was a good immunogen and a strongly reacting antigen in vitro; in Immunoelectrophoresis, each strain migrated as a single antigenic component towards the cathode. The cymbidium and clover strains were serologically closely related, although spurs were produced in immunodiffusion. No serological relationship was found to forty‐three other isometric viruses, including eighteen tombusvirus isolates; CyRSV nevertheless shares many properties with tombusviruses, and we assign it provisionally to this group. The cryptogram is: R/r:1:7/15:S/S:S/O.