Characterization of a Tymo-like Virus Common in Poinsettia

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Abstract
A virus commonly present in commercially grown Euphorbia pulcherrima was transmitted mechanically and by grafting to E. cyathophora and several other euphorbiaceous species and was tentatively designated poinsettia mosaic virus (PMV). No hosts were found except Euphorbia spp. and these were relatively insusceptible; no techniques were found which permitted 100% infection. The thermal inactivation point of PMV was between 60.degree.-65.degree. C; the dilution end point was beyond 10-4. PMV withstood aging at 24.degree. C for 8 days, but not for 10 days. It was purified by the same method as for tobacco streak virus, involving clarification with hydrated calcium phosphate, high-speed centrifugation and acidification to precipitate host material. When centrifuged in sucrose density gradients, 2 zones appeared, the uppermost (50S) being empty or nearly empty particles. Only the lower zone (112S) was infectious and it consisted of well-defined particles .apprx. 25 nm in diameter which closely resembled those of turnip yellow mosaic virus. The A260/280 ratio of unfractionated virus was 1.50, the extinction coefficient (1 mg/ml, 1 cm, 260 nm) was 8.4, and the buoyant density of the bottom component was about 1.43 g/ml. Properties and morphology resembled those of tymoviruses, but no cross-reactions were obtained with antisera to 12 tymoviruses. PMV often was detectable in crude poinsettia sap by agar gel double diffusion tests, but the ELISA [enzyme locked immunosorbent assay] technique was far more reliable. Poinsettias with no obvious symptoms often contained PMV.

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