National survey of viruses in pastures of subterranean clover. I. Incidence of four viruses assessed by ELISA

Abstract

A nationwide survey was made of the time-course incidence of alfalfa mosaic virus (AMV), clover yellow vein virus (CYVV), subterranean clover mottle virus (SCMoV) and subterranean clover red leaf virus (SCRLV) in improved pastures in southern regions of Australia over a 3-year period. Up to 2 500 samples (cvv. Mt Barker or Woogenellup) were collected in winter and spring of 1984, 1985 and 1986 in each of New South Wales, Victoria, South Australia. Western Australia and Tasmania, and assayed by ELISA. Usually, each sample consisted of one or two plants or parts thereof. However, a small number of samples in winter contained parts of up to 10 seedlings. For this reason, incidences are expressed as percentage of infected samples rather than percentages of infected plants. All four viruses occurred in all five states. Averaged over all states, the incidence of infected samples increased progressively from 15.3 to 19.9 to 22.5% from 1984 to 1985 to 1986, respectively, in spring, and from 6.8 to 13.8 to 18.8% respectively in winter. The incidence of samples infected with multiple infections (2-4 viruses) increased from 1.9 to 3.2 to 5.0% from 1984 to 1985 to 1986, respectively, in spring, and from 0.5 to 1.6 to 4.0%, respectively, in winter. Since 1-4 viruses were detected in each infected sample, more individual infections than infected samples were recorded. For example, the incidence of virus infections per paddock increased progressively from 17.5 to 24.1 to 28.6% from 1984 to 1985 to 1986, respectively, in spring. On a state basis, the incidence of infected samples was greatest in W.A., reaching 38.9 and 34.9% in spring of 1985 and 1986, respectively, and in S.A. reaching 30.3% in spring of 1986; these high levels were due largely to SCMoV which reached 97% in one paddock in each of the two states. In N.S.W., Vic. and Tas., the highest incidences of infected samples in either winter or spring were 21.4, 16.2 and 23.1%, respectively. Averaged over all states, the highest mean incidence recorded for samples infected with individual viruses in either winter or spring was 9.4% for AMV, 5.7% for CYW, 10.9% for SCMoV and 7.5% for SCRLV. For AMV and SCRLV, there was an increasing trend from spring 1984 to spring 1986. A similar increasing trend for SCMoV was more evident in winter than in spring. For CYVV, no time-course pattern was evident. These results support the proposition that viruses contribute significantly to 'clover-decline', a well-known problem in pastures of subterranean clover.