Studies On Viruses Infecting Cucurbitaceous Crops

The production of cucurbitaceous crops is increasing recently in Zhejiang Province and other parts of south-east China, where the agricultural economy is relatively well developed, and the crop system is become more flexible. Viral diseases occur in all the area where cucurbits are planted. But the research work on viruses infecting this family of plant remain behind in compare with that for other crops. In this report, viral pathogens naturally occurred on cucurbitaceous crops were isolated and identified systemically. We found that Zucchini yellow mosaic virus (ZYMV) and Cucumber mosaic virus (CMV) were both principal viruses infecting cucurbits in Zhejiang Province and many other parts of Mainland China. Based on this, we studied the genomic diversity of the above two viruses and their seasonal occurrence. The pathogenicity of ZYMV and CMV was also investigated and crop resistance, for different varieties was also compared for against these viruses.Based on host reaction,"double-stranded RNA analysis, viral particle purification and morphological observation, thirteen virus isolates were identified as CMV, sixteen as ZYMV. one as Tobacco mosaic virus (TMV) and other four as unidentified. Infected cucurbit crops exhibited severe mosaic, distortion, stunt, and lump formation on fruits. The symptom usually become more severe when infected with more than one virus.ZYMV isolates obtained from Mainland China, were found to produce local infection to Chenopodiaceae and Fabaceae species tested, and produced systemic infection to plants of Cucurbitaceae but no infection to Solanaceae. Their virus particles are of filamentous shape of about 750 nm in length, and the coat proteins (CP) are about 36.0 kDa when tested with SDS-PAGE. Cucurbits infecting CMV were found to produce local infection to Chenopodiaceae and Fabaceae species tested, and produced systemic infection to plants of both Cucurbitaceae and Solanaceae. Their virus particles are of spherical morphorlogy of about 29 nm in diameter, and the coat proteins (CP) are about 24.0 kDa.For understanding the genomic variation, geological distribution and host adoption of ZYMV the 3' end sequence of its genome including the coat protein (CP) gene and 3' endun-translated region (UTR) were compared, after RT-PCR and cloning of the part genome. Nine Mainland isolates originally isolated from naturally infected Cucurbita moschata , Cucumis melo, Benincasa hispida and B. hispida var. Chieh-qua, were collected and identified from Zhejiang Province, Hainan Province, Shangdong Province, Shanxi Province and Beijing, respectively. The coat protein genes of all the Mainland-China isolates consisted of 837 nucleotides (nt) coding for 279 amino aids. Their 3' end UTR region composed of 210 to 211 nt not including the poly (A) tail. The above sequence data obtained in our study was then compared with previously reported sequences of 16 other ZYMV isolates obtained from other part of the word. In certain degree, similarity of nucleic acid sequence for CP gene was found being related with the host and geological origination, but not very obvious. Similarity of the CP amino acid sequences deduced from nucleic acid of the 25 ZYMV isolates reached a higher sequence similarity and a clearer relationship to the host and geological origination. The relationship extent of sequence similarity to host origin is higher than its extent to geological origin. According to sequence similarity, the amino acid sequence of ZYMV CP was divided into two parts-"the high variable region", containing about 41 amino acids at its N end, and "the conservative region", containing coat protein core-region and C terminal amino acids. The 25 ZYMV isolates could be classifiedinto five genotypes according to "the conservative region", with a 95% sequence similaritywithin each genotype. The 3' end UTR of ZYMV was relatively conservative but some isolates were relatively longer. It was hard to differ ZYMV isolates into genotypes according to the UTR. The above results showed the trend of ZYMV variation...