The effect of dilution on necrotic ringspot virus infectivity and the enhancement of infectivity by noninfective virus

Abstract

The infectivity dilution curves of sour cherry necrotic ringspot (NRSV) and prune dwarf (PDV) viruses on local-lesion hosts were significantly steeper than a theoretical one-hit curve. The slope of the curve varied with the host; on Dolichos biflorus each twofold dilution of NRSV resulted in a three- to fourfold decrease in infectivity. On Momordica balsamina twofold dilution resulted in eight- to tenfold decreases in infectivity. Dilution curves of other viruses on these hosts resembled typical one-hit curves. Supplementing dilute NRSV extracts with additional serologically related virus caused disproportionate increases in infectivity, but the serologically unrelated PDV had no effect. A strain of NRSV noninfective to D. biflorus markedly increased infectivity when added to dilute extracts of a strain causing lesions on this host. UV- or heat-inactivated virus also increased infectivity of dilute extracts of active virus, but virus inactivated by formalin or freezing did not. These results suggest that a dose of two or more particles of NRSV at an infection site is necessary to initiate infection and that the dose requirement may be satisfied partly by virus that is unable to infect.