PRELIMINARY INVESTIGATION OF THE CAUSAL AGENTS OF A DISEASE-CAUSING LEAF CURL OF TOBACCO IN SOUTH-AFRICA

A high incidence (86%) of potyvirus infection was noted in tobacco pla nts exhibiting a form of leaf curl in South Africa. Despite leaf curl being reported in the literature to be of geminiviral aetiology, no ge rminiviruses were detected. Furthermore, no other virus particles were detected by virus purification, TEM and serology. Twelve species of d sRNA were consistently isolated from these tobacco plants, but were ab sent from other forms of leaf curl-affected and healthy tobacco. Aphid and mechanical inoculation demonstrated that the purified potyvirus(e s) did not cause leaf curl symptoms, but rather mild mottle and mosaic symptoms in tobacco, Partial characterization of the potyvirus prepar ation showed a possible relationship to a South African strain of pota to virus Y. Because potyvirus-inoculated plants did not manifest leaf curl symptoms, and because leaf curl symptoms wet-e noted in some plan ts not infected with a potyvirus, it was concluded that the potyvirus is not involved in the led cml aetiology, but causes a latent infectio n, the symptoms of which are masked. The pattern of the dsRNA banding, induction of enations and lack of mechanical and seed transmission ar e common to plant reoviruses. The possibility elf a phytoreovirus invo lvement in this form of leaf curl is currently being investigated. The results from this study suggest that tobacco leaf curl disease worldw ide, with regard to geminiviruses, be re-evaluated.