First report of Verticillium wilt of melon caused by Verticillium dahliae in Tunisia

Wilting melon plants (Cucumis melo) were observed in several greenhouses in Chott Mariem and Souassi regions, in the eastern part of central Tunisia, during the early spring of 2006. Diseased plants exhibited leaf chlorosis followed by typical V-shaped marginal and interveinal yellowing (Fig. 1), necrosis and dropping of leaves (Fig. 2). As affected plants approached physiological maturity, the above ground parts became desiccated and died (Fig. 3). Internal, vascular discoloration in diseased plants extended from the base of the stem upward.

Pure colonies of fungi were consistently and readily isolated from symptomatic stem vascular tissue when cultured on potato dextrose agar (PDA) medium. A Verticillium species was the only fungus isolated and it grew from most plant pieces. Single spore isolates were obtained and identified as Verticillium dahliae on the basis of microsclerotium production (Hawksworth & Talboys, 1970).

Pathogenicity tests were carried out using the root-dip inoculation. Five Verticillium isolates were tested on seedlings of the melon cultivar ‘Ananas d’Amérique’ at the one-leaf stage. Wounded roots were submerged for 30 min in a conidial suspension (1x10⁷ conidia per ml), while control plants were similarly submerged in sterile tap water. Seedlings were transplanted into pots containing a sterile 2:1 mixture of peat/perlite (v/v) and maintained in a growth chamber at 23±2°C (12 h photoperiod). The tested isolates were found to cause wilting and interveinal yellowing and necrosis on melon plants 30 days after inoculation. V. dahliae was successfully re-isolated from the stems of the inoculated plants.

Occurrence of Verticillium wilt of melon caused by Verticillium dahliae has been reported from the Mediterranean region, Europe and USA (Pegg & Brady, 2002) but this is the first report from Tunisia.