The distribution of poplar mosaic virus in hybrid poplars and virus detection by ELISA

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Abstract
SUMMARY: Purified poplar mosaic virus (PMV) at a concentration of 8 ng/ml was readily detected by enzyme‐linked immunosorbent assay (ELISA). Bioassay in Nicotiana megalosiphon was more sensitive (detecting 1–4 ng/ml) and latex flocculation less sensitive (c. 25 ng/ml) than ELISA assays. While the foliar sap of fresh, naturally‐infected poplars (e.g. Populus. euramericana cv. Robusta) was not infective at dilutions greater than 2 . 10–2, ELISA easily detected PMV antigen when sap was diluted 4 . 10–3 in buffer or when one part of infected tissue was triturated with 99 parts healthy leaf. Furthermore, although sap from poplar leaves stored at ‐20 °C for 6 months was not infective, PMV was still detectable in ELISA tests. PMV antigen in poplar leaves was not all pelleted after centrifugation for 2.5 h at 130 000 g yet parallel tests using unbuffered sap from systemically infected Nicotiana megalosiphon foliage showed that infectivity was restricted to the pellet. In poplar foliage, the concentration of PMV antigen was generally greatest where symptoms were most obvious; least antigen was detected in the overwintering leaves located at the bases of long shoots. In winter, when root and inner bark tissue in the trunk was an erratic source of PMV, the virus was readily detected in buds, the concentration being greatest in the bases, including the meristem, of terminal buds.Propagation from single node cuttings of P. euramericana cv. Regenerata allowed the selection of clones that consistently showed either ‘severe’ or ‘mild’ foliar symptoms. The associated virus isolates also infected another poplar clone causing symptoms characteristic of their source. ELISA consistently detected less PMV antigen in field‐grown cv. Regenerata than in cv. Robusta foliage, but this was reversed when the associated virus isolates were propagated in Nicotiana glutinosa at 24 °C.During 6 yr, 21 out of 127 poplars at a site in Western England, became infected with PMV. By contrast, in Eastern England, none of 46 were infected. The aphids Pterocomma populea and Myzus persicae did not transmit PMV.