Characteristics of clover primary leaf necrosis virus, a new spherical isolate from Trifolium pratense

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Abstract
An unknown virus was isolated from a young red clover plant (Trifolium pratense) with a bright yellow leaf mottle and subsequently was isolated from five other field clover plants with milder symptoms growing in three locations. In the laboratory, red clover became systemically infected by the virus only when the plants were kept between 10 and 16 °C after inoculation, and symptoms were mild. Crimson clover (T. incarnatum) was readily invaded at room temperature, and survivors of the initial shock reaction were severely mottled. White clover (T. repens) and Alsike clover (T. hybridum) did not become systemically infected under either temperature regime. The symptom common to all four species, necrotic spots in the inoculated primary leaves, suggested the name clover primary leaf necrosis virus. Among the nine leguminous and six non-leguminous host species, bean (Phaseolus vulgaris) was best suited for virus propagation, and cucumber (Cucumis sativus) was best suited for quantitative assay and detection.The virus was characterized by a single sedimenting species of spherical nucleoprotein particles with a sedimentation value, S20.w, of 136–137, an average diameter of 36 nm, and a specific extinction, E260 nm1%, 1 cm, of 58.15. The nucleic acid was of the ribose type and constituted 21% of the weight of the virion. Activity was lost from crude juice at 65 °C and from purified suspensions at 85 °C, with about 10% activity persisting between 60–70 °C. Two electrophoretic components were isolated from purified preparations. They induced identical symptoms in three hosts, but one replicated both components in bean and had more antigenic determinants than the other, which replicated itself only. The virus was weakly antigenic inducing an antiserum with titer of 128. Some of its in vitro properties were similar to those of carnation ringspot virus, but the two viruses were serologically unrelated. Nor was this virus serologically related to any of 15 other spherical viruses tested.

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