Evidence of synergism between African cassava mosaic virus and a new double-recombinant geminivirus infecting cassava in Cameroon

Stem cuttings were collected in Cameroon from cassava plants displaying cas sava mosaic disease (CMD) symptoms. The nature of the viruses present was d etermined by using the PCR with primers specific for the coat protein (CP) genes of African cassava mosaic virus (ACMV) and East African cassava mosai c virus (EACMV). All samples were infected by ACMV and eight of the 50 samp les were infected by both ACMV and an EACMV-like virus. The complete nucleo tide sequences of DNA-A and -B of representative ACMV and EACMV-like viruse s were determined. The DNA-A component of the EACMV-like virus contained ev idence of recombination in the AC2-AC3 region and DNA-B also contained evid ence of recombination in BC1. However, both components retained gene arrang ements typical of bipartite begomoviruses. When Nicotiana benthamiana plant s were doubly inoculated with these Cameroon isolates of ACMV and EACMV (AC MV/CM, EACMV/CM) by using sap from cassava plants or infectious clones, the symptoms were more severe than for plants inoculated with either virus alo ne. Southern blot analysis of viral DNAs from infected plants showed that t here were significantly higher levels of accumulation of both ACMV/CM compo nents and, to a lesser extent, of EACMV/CM components in mixed-infected pla nts than in singly infected plants. These results strongly suggest the occu rrence of a synergistic interaction between the two viruses.