A distinct tymovirus infecting Cassia hoffmannseggii in Brazil.

Leaves of Cassia hoffmannseggii, a wild fabaceous species found in the Atlantic Forest, with a severe mosaic symptom were collected in Pernambuco State, Brazil. By transmission electron microscopy, two types of virus particles were found: the first was recognized as particles of a potyvirus, which was later identified as Cowpea aphid-borne mosaic virus; and the second was isometric and present in high concentration. The observation of vesicles at the periphery of chloroplasts suggested a tymovirus infection, which was confirmed by subsequent assays. A serological assay against several tymovirus antisera resulted in positive reaction of this tymo-like virus with an antiserum of Passion fruit yellow mosaic virus. By means of RT-PCR and using degenerated primers for the conserved region of RNA-dependent RNA polymerase (RdRp) gene of tymoviruses, a specific DNA fragment was amplified and sequenced. Based on this sequence, a specific forward primer was synthesized and successfully used to amplify the 3' terminal genome region, containing the partial RdRp gene and the complete coat protein (CP) sequences. The CP was 188 amino acids (aa) long, and the highest CP aa identity was observed with Kennedya yellow mosaic virus (61 %). Based on the current ICTV demarcation criterion, this isolate was considered as a distinct tymovirus and tentatively named as Cassia yellow mosaic-associated virus.

Occurrence of Cowpea aphid-borne mosaic virus in Peanut in Brazil.

Surveys of peanut crops in northeastern Brazil since 1995 showed the occurrence of a hitherto unreported virus disease. Characteristic leaf symptoms were ring spots and blotches. The virus was seed transmitted in peanut (1/610) and cowpea (47/796). Local and systemic symptoms were observed in cowpea (cv. TVu 3433) known to be susceptible to most Cowpea aphid-borne mosaic virus (CABMV) isolates. The virus was transmitted by aphids Toxoptera citricidus and Aphis gossypii. Using degenerate primers, the 3' terminal region of the viral genome was cloned and sequenced. Sequence analyses of the coat protein and the 3' untranslated region indicated that the potyvirus was most closely related to CABMV isolates from South Africa, Zimbabwe, and the United States. On the basis of genome analysis, the virus was identified as CABMV. The natural occurrence of CABMV on peanut has so far not been reported. The significance of this finding especially for germ plasm exchange is discussed.

RT-PCR Detection of Seedborne Cowpea aphid-borne mosaic virus in Peanut.

The Brazilian strain of Cowpea aphid-borne mosaic virus (CABMV) is a severe pathogen in peanut and a significant problem when distributing germ plasm to other countries. The virus is seedborne at approximately 0.15% in peanut, depending upon the cultivar, and its detection in seed lots would strengthen quarantine programs. Utilizing 3' sequence data (GenBank Accession #AF241233), primers were designed from the coat protein region and evaluated by reverse transcription-polymerase chain reaction (RT-PCR). Use of the forward primer 5'-CGCTCAAACCCATTGTAGAA-3' and reverse primer 5'-TATTGCTTCCCTTGCTCTTTC-3' yielded a 221-bp product. Extracts of thick seed slices and a sample size of 12 to 25 seed showed no significant advantage of RT-PCR over enzyme-linked immunosorbent assay (ELISA) in tests of large seed lots. However, RT-PCR detected more virus in seed than in the number of infected seedlings normally arising in germination tests. Also, RT-PCR was extremely sensitive and detected 1 infected leaf among 99 healthy leaves. In contrast, ELISA detected only one infected leaf among nine healthy leaves.

Cowpea aphid-borne mosaic virus (CABMV) is widespread in passionfruit in Brazil and causes passionfruit woodiness disease.

Leaf samples of yellow passionfruit (Passiflora edulis f. flavicarpa) displaying fruit woodiness symptoms were collected in seven Brazilian states and the Federal District. Viral infection was confirmed by host range and ELISA, and fourteen viral isolates were obtained. All isolates were capable of infecting several leguminous host species, although differences in symptom severity were noticeable. Woodiness symptoms were reproduced in yellow passionfruit, and mosaic symptoms were induced in common bean. All isolates infected cowpea, reported as a non-host of passion fruit woodiness virus (PWV). Indirect ELISA demonstrated that all isolates were serologically related to each other and also to cowpea aphid-borne mosaic virus (CABMV). The complete sequence of the capsid protein was determined for all isolates. Comparison of these sequences with those of other potyviruses indicated the highest identity with CABMV isolates (85 to 94%). Identity with PWV isolates ranged from 54 to 70%. Phylogenetic analysis grouped all of the Brazilian isolates in a monophyletic cluster with the CABMV isolates, clearly distinct from the PWV isolates. Furthermore, this analysis demonstrated that a group of previously characterized isolates from Brazil that had been designated as PWV should be reclassified as CABMV. Together, these results provide unequivocal evidence that, in Brazil, passionfruit woodiness disease is primarily caused by CABMV. The presence of PWV in Brazil has yet to be confirmed.