Geminivirus isolates associated with the epidemic of severe cassava mosaic disease in Uganda were studied and compared with virus isolates from the part of Uganda outside the epidemic area, and with African cassava mosaic virus (ACMV) and East African cassava mosaic virus (EACMV). Isolates of a novel type [the Uganda variant (UgV

The history, symptoms, distribution and prevalence, aetiology, epidemiology and control of cassava viral diseases from Africa, South/Central America and Asia/Pacific are discussed.

Calvert, L. A. and Ghabrial, S. A. 1983. Enhancement by soybean mosaic virus of bean pod mottle virus titer in doubly infected soybean. Phytopathology 73:992-997.

The 3'-terminal region of wheat streak mosaic virus (WSMV) genomic RNA was cloned and a cDNA sequence of 1809 nucleotides upstream of the poly(A) tract was determined. The sequence contains a single open reading frame of 1662 nucleotides and a 3' untranslated region of 147 nucleotides. Translation products from WSMV RNA and WSMV cDNA transcripts were immunoprecipitated by WSMV capsid protein antiserum, indicating that the 3'-terminal region of WSMV RNA encodes the capsid protein.Five potential N-terminal capsid protein protease cleavage sites were identified, which would yield proteins ranging from 31-7K to 46.8K. Alignment of the deduced amino acid sequence of the WSMV capsid protein with those of other potyviruses showed significant, but limited, identity as compared to the alignment of two or more aphid-transmitted potyviruses. Although WSMV has characteristics distinct from potyviruses, because of its particle morphology, translation strategy apparently based on polyprotein processing, the ability to form cytoplasmic cylindrical inclusions and the degree of capsid protein homology with aphidtransmitted potyviruses, it should be considered a member of the potyvirus group.

No abstract

No information exists on the organization and mechanisms of expression of the genome of rice hoja blanca virus (RHBV), a member of the tenuivirus group, but here we describe the first steps in its characterization. RHBV contains four ssRNA and three dsRNA species, the sizes of which were estimated by native and denaturing gel electrophoresis. Hybridization analyses using 32p-labelled riboprobes of viral and viral complementary polarities showed that unequal amounts of the two polarities of at least the smallest RNA are present in the virion, and indicated that the dsRNA species contain the same information as the ssRNA species of corresponding size. Total RHBV RNA directs the synthesis of two major proteins of 23K and 21K in vitro. RNA3 directs the synthesis of a 23K protein designated NS3, and RNA4 of a 21K protein designated NS4. The NS4 protein corresponds to the non-structural protein that accumulates in RHBV-infected rice tissue. The nucleocapsid protein is not translated from either total RHBV RNA or any individual RHBV RNA in vitro.

Cassava vein mosaic virus (CVMV) was found to be widespread throughout the north-eastern region of Brazil. The complete sequence of CVMV was determined, and the genome was 8158 bp in size. A cytosolic initiator methionine tRNA (tRNAn~t.0-binding site that probably acts as a primer for minus-strand synthesis was present. The genome contained five open reading frames that potentially encode proteins with predicted molecular masses of 186 kDa, 9 kDa, 77 kDa, 24 kDa and 26 kDa. The putative 186 kDa protein had regions with similarity to the zinc finger-like RNAbinding domain that is a common element in the capsid proteins and similarity to the intercellular transport domain of the plant pararetroviruses. The predicted 77 kDa protein had regions with similarity to aspartic proteases, reverse transcriptase and RNase H of pararetroviruses. This gene order was confirmed by the amplification of similar PCR products from total DNA extracted from CVMV-infected cassava plants. The genomic organization of CVMV was different from the organization of either the caulimoviruses or badnaviruses. In comparisons of the regions with the reverse transcriptase motif, CVMV was grouped between the caulimoviruses and badnaviruses. It appears that CVMV is distinct from the other well-characterized plant pararetroviruses.

Cassava Frogskin Disease (CFSD) can cause severe damage to cassava roots and is one of the most important diseases of cassava in Latin America. The principal objective of this study was to identify the causal agent of CFSD. Electron microscopy, viral purifications, double-stranded RNA (dsRNA) analysis, cloning, sequencing, rtPCR and hybridizations were carried out to characterize and associate a novel virus with the disease. Virus-like particles of 70 and 45 nm in diameter were found in affected cassava plants and partially purified preparations respectively. Nine species of dsRNA were associated with this disease and cDNA clones to six genomic segments were synthesized from the purified dsRNAs. The putative proteins predicted from the sequence of the cassava virus cDNA clones have similarity with the P1, P2, P3, P4, P5 and P10 proteins of Rice ragged stunt virus (RRSV). Phylogenic analysis confirmed that this virus is a member of the family Reoviridae and is most closed related to RRSV. Hybridization analyses of dsRNA identified S1, S2, S3, S4, S5 and S10 genomic segments in the CFSD-affected plants, but not in healthy controls. Additionally, 26 isolates were compared using a portion of the putative polymerase gene. The virus was detected in all 26 isolates, and they were classified into three distinct races. The association of this virus with CFSD was strengthened by the detection of this virus in diseased plants collected from different locations throughout Colombia.