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Evidence for a Phytoreovirus Associated with Tobacco Exhibiting Leaf Curl Symptoms in South Africa

April 1999 , Volume 89 , Number  4
Pages  303 - 307

Marie Emma Christine Rey , Elvera D'Andrea , Jennifer Calvert-Evers , Maria Paximadis , and Guido Boccardo

First, second, third, and fourth authors: Department of Microbiology, University of the Witwatersrand, P.O. Wits 2050, Johannesburg, South Africa; fifth author: CNR, Istituto di Fitovirologia Applicata, Str. delle Cacce 73, 10135 Torino, Italy

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Accepted for publication 13 January 1999.

Three forms of tobacco leaf curl (termed classes I, II, and III, based on symptomatology) recently have been described in southern Africa. Numerous attempts to isolate virus particles responsible for a nongeminivirus-induced leaf curl disease (class I) of tobacco in South Africa have been unsuccessful. Recently, 12 dsRNA segments were isolated from tobacco exhibiting class I leaf curl symptoms, suggesting a possible reovirus genome. The objective of our study was to confirm whether the dsRNA segments are associated with a reovirus. Isolation of icosahedral particles with an outer core 60 to 65 nm in diameter and an inner core 40 to 45 nm in diameter was achieved. Twelve distinct nonpolyadenylated dsRNAs were isolated from purified virions, and the total molecular masses of the dsRNAs ranged from 17.86 to 18.40 × 106 Da in polyacrylamide and agarose gels, respectively. Using hybridization analysis, dsRNAs were identified as non-homologous distinct segments. Comparisons with other known reoviruses revealed a unique banding pattern that was most similar to the wound tumor virus (WTV), the type species of the genus Phytoreovirus. Hybridizations of WTV cloned DNA probes (segments S4 and S6 to S9) and dsRNAs from infected tobacco indicated no significant sequence similarity, whereas indirect enzyme-linked immunosorbent assay with a polyclonal antiserum to WTV showed strong positive cross-reactivity to tobacco virions. Our results indicate a virus with features consistent with those of phytoreoviruses. This is the first report of a plant reovirus in tobacco, the first record in Africa, and the second example of a field-isolated dicot phytoreovirus.

© 1999 The American Phytopathological Society