First report of Melon necrotic spot virus on melon in Tunisia

During April 2006, melon (Cucumis melo) plants showing necrotic spots on leaves, occasionally followed by death of affected plants, were observed in commercial, plastic houses heated by geothermic water in Kebili (southern Tunisia). Crude sap from leaves with symptoms was used for mechanical inoculation onto a set of differential host plants. Necrotic local lesions on inoculated leaves or cotyledons and systemic necrotic spots were observed on melon plants. Necrotic local lesions and no systemic symptoms were produced on cucumber (Cucumis sativus). In contrast, no symptoms were observed on non-cucurbit plants.

The observed symptoms and host range were similar to those described for Melon necrotic spot virus (MNSV). The presence of MNSV was established by DAS-ELISA using a specific antiserum prepared at INRA. MNSV was detected in seven samples with symptoms. Total RNA was extracted using Tri-Reagent and tested using Takara One-Step RNA PCR kit with specific primers MNSV-CP-5' GTGAAGCTTAARCAGGC and MNSV-CP-3' ACRTARAGATCACCRTGGGC, designed to amplify a region of the coat protein gene. RT-PCR products of the expected size of 711 bp were generated from infected plant extracts. No amplicon was produced from healthy plant extracts.

RT-PCR product from isolate TN06-14 was directly sequenced (GenBank EF442681). This nucleotide sequence showed 96%, 94% and 92% identity to the coat protein gene of MNSV isolates from Spain (GenBank DQ339157), the Netherlands (GenBank M29671) and Panama (GenBank DQ443547) respectively. MNSV is endemic in many melon and cucumber production areas; it has been reported in the Americas, Japan and Europe (Brunt et al., 1996). It is transmitted by the fungus Olpidium bornovanus and through the seed with the assistance of its vector (Campbell et al., 1996). Its prevalence in protected crops in Tunisia might be related to intensive and repeated melon cultivation in plastic houses (Blancard et al., 1991). To our knowledge, this is the first report of MNSV in Tunisia or indeed in any African country.