First Report of Cucurbit aphid-borne yellows virus in Tunisia Causing Yellows on Five Cucurbitacious Species

Viruses, distributed worldwide on cucurbits, cause severe damage to crops. Virus surveys in 2003 and 2004 were made in all the major cucurbit-growing areas in Tunisia. Large populations of aphids (Aphis gossypii Glover) and severe yellowing symptoms of older leaves of cucurbits were observed in outdoor and under plastic-tunnel cultivation, suggesting the presence of Cucurbit aphid-borne yellows virus (CABYV, genus Polerovirus, family Luteoviridae). Leaf samples collected from symptomatic and asymptomatic plants of melon (Cucumis melo L.), cucumber (C. sativus L.), squash (Cucurbita pepo L.), watermelon (Citrullus lanatus L.), and ware cucurbit (Ecballium elaterium L. T. Richard) were screened for the presence of CABYV using enzyme-linked immunosorbent assay (ELISA) and reverse transcription-polymerase chain reaction (RT-PCR). Reference isolate, CABYV-N (GenBank Accession No. X76931) was provided by H. Lecoq (INRA-Monfavet Cedex, France). Sample extracts from fresh leaf tissues were tested using ELISA with an antiserum prepared against this isolate. In addition, total RNA was extracted from fresh leaf tissues according to the technique of Celix et al. (2) using the Titan RT-PCR kit from Roche Diagnostics (Penzberg, Germany). Forward primer (5'-GAGGCGAAGGCGAAGAAATC-3') and reverse primer (5'-TCTGGACCTGGCACTTGATG-3') were designed with the available sequence of the reference isolate. ELISA tests demonstrated that 91 plants were positive among 160 plants tested with severe yellowing symptoms. All asymptomatic plants were negative. RT-PCR results yielded an expected 550-bp product that was amplified from the reference isolate. Of the 160 plants tested using ELISA, 106 plants were screened with RT-PCR including the 91 plants that were positive in ELISA. These 91 plants also were positive after RT-PCR amplification as were 12 more plants. This demonstrated that the RT-PCR test is more sensitive. No amplicons were produced from extracts of asymptomatic plants, RNA preparations of Cucurbit yellow stunting disorder virus (CYSDV), or Beet pseudo yellows virus (BPYV) positive controls provided by B. Falk (University of California, Davis). CYSDV and BPYV can induce similar yellowing symptoms in cucurbits. The results of the ELISA and RT-PCR tests showed that CABYV is widely distributed on five cucurbit species in the major growing areas of Tunisia including the northern, Sahel, central, and southern regions where it was detected, respectively, in 10 of 25, 11 of 21, 24 of 37, and 58 of 77 samples tested. CABYV was detected at the rates of 63 of 72 on melon, 10 of 21 on cucumber, 17 of 24 on squash, 10 of 25 on watermelon, and 3 of 18 on ware cucurbit. CABYV also seems to be widespread throughout the Mediterranean Basin (1,3,4), but to our knowledge, this is the first report of the occurrence of CABYV in Tunisia on different species of cucurbit and ware cucurbit. References: (1) Y. Abou-Jawdah et al. Crop Prot. 19:217, 2000. (2) A. Celix et al. Phytopathology 86:1370, 1996. (3) M. Juarez et al. Plant Dis. 88:907, 2004. (4) H. Lecoq et al. Plant Pathol. 41:749, 1992.