Snake melon asteroid mosaic virus, a Tentative New Member of the Genus Sobemovirus Infecting Cucurbits.

A virus isolate (Su-95-67) was obtained from a snake melon (Cucumis melo var. flexuosus) plant presenting severe chlorotic spots, mosaic, stunting, and leaf deformations collected in Eastern Sudan in 1995. Su-95-67 was easily mechanically transmissible and had a host range limited to a few cucurbit species. Isometric virus particles approximately 30 nm in diameter were observed in leaf dip preparations. A cytopathological study did not reveal alterations specific for a virus genus or family. A polyclonal antiserum was obtained and used in double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA). Su-95-67 was transmitted by seed at a low rate, by the red melon beetle (Aulacophora foveicollis), but not by the melon aphid (Aphis gossypii). Because Su-95-67 shared several properties with sobemoviruses, generic Sobemovirus reverse-transcription polymerase chain reaction primers were developed. They allowed amplification of a 384-bp fragment from extracts of plants infected by two sobemoviruses or by Su-95-67 but not from healthy plants extracts. Sequence comparison confirmed that Su-95-67 belongs to a new tentative Sobemovirus species for which we propose the name Snake melon asteroid mosaic virus (SMAMV). DAS-ELISA tests conducted on extracts of virus-infected cucurbit plants collected from 1992 to 2003 revealed the presence of SMAMV in 10.2% of 600 samples originating from different regions of Sudan.

Interaction between potyvirus helper component-proteinase and capsid protein in infected plants.

Monoclonal antibodies were raised against helper component-proteinase (HcPro) purified from plants infected with the potyvirus Lettuce mosaic virus (LMV). These antibodies were used in a two-site triple antibody sandwich ELISA assay together with polyclonal antibodies directed against purified virions. An interaction between HcPro and the viral coat protein (CP) was demonstrated in extracts of LMV-infected leaves, as well as for two other potyviruses, Plum pox virus and Potato virus Y. The CP-HcPro interaction was not abolished in LMV derivatives with an HcPro GFP N-terminal fusion, or with a deletion from the CP of the amino acids involved in aphid transmission. Electron microscopy indicated that HcPro probably does not interact with the CP in the form of assembled virions or virus-like particles. Together, these results suggest that the interaction detected between CP and HcPro might be involved in a process of the potyvirus cycle different from aphid transmission.

Biological and Molecular Characterization of Moroccan watermelon mosaic virus and a Potyvirus Isolate from Eastern Sudan.

A potyvirus (Su-94-54) was isolated from a naturally infected snake cucumber (Cucumis melo var. flexuosus) plant with severe mosaic and leaf deformation symptoms collected in Eastern Sudan. This isolate has a host range limited to cucurbits and is serologically distantly related to Moroccan watermelon mosaic virus (MWMV) and to Papaya ringspot virus (PRSV). Coat protein sequence analysis of Su-94-54 and MWMV and comparison with other potyviruses indicate that Su-94-54 is more closely related to MWMV than to any other potyvirus. Based on the amino acid sequence identity in the core part of the coat protein with MWMV (86%), this isolate could be regarded as a distinct species. However, because of biological, cytological, and serological affinities with MWMV, we propose that this isolate be considered as a strain of MWMV, possibly an evolutionary intermediate between MWMV and PRSV, until more is known on the structure of the PRSV subgroup within the genus Potyvirus.

Effects of Onion Yellow Dwarf and Leek Yellow Stripe Viruses on Symptomatology and Yield Loss of Three French Garlic Cultivars.

This study was conducted to determine the effect of two potyviruses, onion yellow dwarf virus (OYDV) and leek yellow stripe virus (LYSV), on the symptoms, growth, and potential yield loss of garlic (Allium sativum). For 2 consecutive years, the impact on leaf length, pseudostem diameter, and bulb weight was evaluated after mechanical inoculation of cultivars Messidrome, Germidour, and Printanor, the three main garlic cultivars grown in France. The reduction in bulb weight due to OYDV ranged from 39% for Germidour to about 60% for the two other cultivars. For LYSV, the reduction in bulb weight was less on Messidrome (17%) and Germidour (26%) than on Printanor (54%). Coinfection with both viruses further reduced growth and bulb weight. When cloves originating from bulbs infected by each virus alone or a mixture of both viruses were planted, results indicated that such chronic infection induced further yield reduction. An assay designed to evaluate the role of LYSV inoculation date on yield revealed that yield losses were the lowest for late-season infections. However, yield loss was greater than 30% when the inoculation was performed at the end of April, the time when natural contamination generally occurs in southern France. A comparison of the impact of mixed infections of OYDV and LYSV from different origins suggested that the isolates did not differ significantly in their effects on yield loss.

Melon Rugose Mosaic Virus: Characterization of an Isolate from Sudan and Seed Transmission in Melon.

Melon rugose mosaic virus (MRMV) was isolated from snake cucumber (Cucumis melo var. flexuosus) in the Kassala region of Sudan in 1993. The host range of the virus was mostly limited to cucurbits, where it induced severe mosaic and leaf deformations. Cytopathological studies revealed severe chloroplast alterations, including vesicles at their periphery and the tendency to aggregate, which are typical of tymovirus infections, providing further evidence that MRMV is a tentative member of the genus Tymovirus. In melon and snake cucumber, MRMV was found to be seed transmitted at rates of 0.9 and 3.8%, respectively. Seed dissection experiments revealed that the virus could be detected in the seed coat, papery layer, and embryo. Seed disinfection treatments did not reduce seed transmission rates, which suggests an internal transmission. A preliminary screening for resistance in melon revealed some resistance in two out of 367 accessions tested.