June 27, 2005
Source: American Phytopathological Society, Plant Disease Notes [edited]

First report of cucurbit aphid-borne yellows virus in Tunisia causing Yellows on 5 cucurbitacious species
M. Mnari Hattab, Laboratoire de Protection des Vegetaux, Institut National de la Recherche Agronomique de Tunis, 2049 Ariana, Tunisia; J. Kummert and S. Roussel, Unite de Phytopathologie, Faculte Universitaire des Sciences Agronomiques, B5030 Gembloux, Belgium; K. Ezzaier, Laboratoire de Protection des Vegetaux, Institut National de la Recherche Agronomique de Tunis, 2049 Ariana, Tunisia; A. Zouba, Pole Regional de Recherche Developement Agricole 2260 Deguache, Tunisia; and M. H. Jijakli, Unite de Phytopathologie, Faculte Universitaire des Sciences Agronomiques, B5030 Gembloux, Belgium. Plant Dis. 89:776, 2005; published on-line as DOI: 10.1094/PD-89-0776B. Accepted for publication 26 Apr 2005.

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 result 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 5 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 1st 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.

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[Cucumber aphid-borne yellows virus (CABYV) is a luteovirus with isometric particles of 25 nm in diameter. The genome of CABYV consists of a single molecule of positive sense ssRNA. It was 1st reported in France (Lecoq et al. 1992). So far, it has been recorded in a few African countries that include Morocco, Sudan, and Zambia
(Lecoq, personal communication). Typical symptoms of CABYV on cucurbits include yellowing and thickening of old leaves. Yield reduction in infected cucumber crops can reach about 50 percent (Lecoq et al. 1992). Major vegetable species susceptible to CABYV include watermelon (_Citrullus lanatus_), muskmelon (_Cucumis melo_),
cucumber (_C.sativus_), zucchini squash (_Cucurbita pepo_), and lettuce (_Lactuca sativa_). CABYV can be transmitted persistently by _Aphis gossypii_ and _Myzus persicae_ (Lecoq et al. 1992). CABYV is not transmitted mechanically.

According to Abou-Jawdah and colleagues at American University, Beirut and INRA, France, viral diseases are the major cause of economic losses in commercial cucurbit production in Lebanon. A survey revealed that Zucchini yellow mosaic potyvirus (ZYMV) and Cucurbit aphid-borne yellows polerovirus (CABYV) are the most common viruses of field-grown cucurbits in Lebanon. Other viruses involved were Watermelon mosaic potyvirus (WMV), Papaya ringspot potyvirus-watermelon strain (PRSV-W) and Cucumber mosaic cucumovirus (CMV).

Inoculation of squash with a mild strain of ZYMV (strain WK) gave significant protection against severe virus symptoms and resulted in significant yield increase as compared to the control, as did silver plastic mulch. The best protection and highest total yield were obtained with floating row covers (flexible nets or other types of screens). Integration of cross-protection with silver mulch or floating row covers improved the protection obtained with either approach. Cross-protection is still a useful disease management option for reducing yield loss and maintaining reasonable quality of product. Resistant curcubit cultivars are not available, but sources of resistance to CABYV have been identified in melon accessions.

Resistance towards CABYV in Sudanese melons was only detected in the wild agrestis melons. Among different melon genetic resources collected from different parts of the world and evaluated for resistance against CABYV, the Sudanese Humaid accession T-EK 92-2 was found segregating for resistance (Dogimont et al. 1996). Further, the wild melon (Humaid) line HSD 2445-005 was found to be homogenously resistant, while the Humaid accession HSD 2441 was also found segregating for CABYV resistance

Other links: <http://www.iita.org/info/virology/pdf_files/213-232.pdf>
<http://www.ipgri.cgiar.org/pgrnewsletter/article.asp?id_article=7&id_issue=138>

Y. Abou-Jawdah, H. Sobh, S. el-Sammar, A. Fayyad, and H. Lecoq. 2000.
Incidence and management of virus diseases of cucurbits in Lebanon.
Crop Protection, Vol. 19 (4) 217-224. - Mod.DH]

[see also in the archive:
2004
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Cucurbit virus diseases - Sudan 20040918.2589
Cucurbit aphid-borne yellows virus - Spain (Murcia) 20040811.2224]