ROLE OF A NOVEL TYPE OF DOUBLE INFECTION IN THE GEMINIVIRUS-INDUCED EPIDEMIC OF SEVERE CASSAVA MOSAIC IN UGANDA

To study the cause of the current epidemic of severe mosaic in Ugandan cassava, PCR analysis was used to detect and identify African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV) and th e recently reported recombinant geminivirus (UgV), which is derived fr om ACMV and EACMV, in leaf extracts from cassava plants grown from cut tings in the glasshouse at Dundee. The cuttings were collected from pl ants showing symptoms of different severities and growing at different sites in Uganda inside, at the periphery of, and outside, the area af fected by the epidemic. ACMV occurred throughout the nine districts sa mpled but UgV was detected only in the area affected by the epidemic. EACMV was not found in Uganda. Most plants containing ACMV alone expre ssed mild or moderate mosaic, whereas very severe mosaic developed in most plants containing UgV plus ACMV and a few of those containing UgV only. Very severe mosaic in cassava from southern Sudan was likewise associated with co-infection by UgV and ACMV. The very severe disease was reproduced by graft-inoculating geminivirus-free cassava with UgV plus ACMV; plants inoculated with either UgV or ACMV developed severe or moderate symptoms, respectively. Unlike ACMV, Malawian EACMV did no t enhance the severity of symptoms induced by UgV. However, a very sev erely affected plant from Ukerewe Island, Tanzania, contained ACMV and EACMV but not UgV. UgV attained a much greater concentration in cassa va than did ACMV but the opposite occurred in Nicotiana benthamiana. I n neither host was total virus antigen concentration affected by co-in fection. Factors affecting the genesis, selection and spread of UgV ar e discussed. The evidence indicates that UgV is probably of relatively recent origin, that such variants do not appear often, and that the c urrent epidemic has resulted from the rapid spread of UgV to infect pl ants and to invade regions in which ACMV already occurred. The novel t ype of virus complex so produced, consisting of an interspecific recom binant virus (UgV) and one of its parents (ACMV), typically has even m ore severe effects than UgV alone.