Résumé
Sugarcane leaf samples from 30 cultivars exhibiting mosaic symptoms and that originated from Bangladesh, India, Pakistan, Sri Lanka, Thailand, and Vietnam did not react in serological assays (ELISA or tissue blot immunoassay) with a Sugarcane mosaic virus (SCMV) antiserurn or with a Potyvirus specific monoclonal antibody. These samples also reacted negatively in RT-PCR tests with SCMV, Sorghum mosaic virus (SrMV), Potyvirus or Potyviridae specific primers. Mosaic symptoms were, however, reproduced after mechanical inoculation of sugarcane (cultivar B46364) and sorghum (cultivar Tx412) plants with diseased leaf extracts. Electron microscopy of partially purified virions from inoculated plants showed flexuous filaments characteristic of the Potyviridae family. Several primer pairs were then designed to detect Sugarcane streak mosaic virus (SCSMV) by RT-PCR. SCSMV was detected by RT-PCR in all samples with mosaic symptoms, and the amplicon (0.5 kb) from the coat protein (CP) coding region of seven isolates was cloned, sequenced and compared with published sequences of SCSMV. This virus, causal agent of streak mosaic, has been reported so far only in Pakistan and India. The CP nucleotide identity for the seven isolates ranged from 97 to 99% with SCSMV from Pakistan (GenBank accession number U75456) and from 93 to 95% with SCSMV from India (GenBank accession number Y17738). Sorghum cultivar Tx412 susceptible to mosaic was inoculated in a growth chamber with two isolates of SCMV, one isolate of SrMV and five isolates of SCSMV. Mosaic symptoms appeared one week after inoculation of plants with SCMV and SrMV, and almost all plants inoculated with these two viruses exhibited severe symptoms after two weeks. Disease severity and disease incidence remained very high in these plants until the end of the experiment (8 weeks). Mosaic symptoms appeared within two weeks after inoculation of plants with some isolates of SCSMV. Symptoms produced by the SCSMV isolates were, however, much less severe than those produced by SCMV and SrMV, whatever the observation date. Our results suggests that SCSMV is the major cause of mosaic symptoms in sugarcane cultivars in several Asian countries. However, recent analysis of additional samples showed that some cultivars with mosaic symptoms from Pakistan and India were infected with both SCSMV and SCMV. (Texte intégral)