First report of Verticillium dahliae race 2 in Tunisia

Vascular wilt diseases have recently become a serious problem on tomato (Lycopersicon esculentum) throughout the Tunisian Sahel. Since 2002, plants of cv. 'Colibri' with symptoms characteristic of Verticillium infection (wilting, yellowing, stunting and dark brown vascular discoloration) have been seen in many greenhouses in the Chott Mariem region (Fig. 1). Isolates from diseased plants were identified as Verticillium dahliae on the basis of microsclerotium production (Hawksworth & Talboys, 1970). The cultivar 'Colibri' carries a single dominant gene (Ve) conferring resistance to race 1 of V. dahliae (Schaible et al., 1951). This resistance has been effective for over 20 years. Wilt caused by V. dahliae has also been confirmed in several other resistant cultivars (e.g. Amel, Cenkara, Rio Grande) (Fig.2). Fifty-one Tunisian isolates were tested to determine their race, with Canadian and Israeli race 1 and 2 isolates (one each) were included as controls. Conidial suspensions of 107 conidia per ml were used to inoculate plants of the cultivars 'Ventura' and 'Sun 6200' (susceptible to both races), and 'Rio Grande', 'Colibri' and 'Naya' (resistant to race 1), at the two-leaf stage by root dipping. Seedlings were replanted into a sterile 2:1 mixture of peat/perlite (v/v) and maintained in a growth chamber at 23 ± 3°C. Equal numbers of plants of each cultivar were dipped into water as controls. The experimental design was a completely randomized type with ten replicates (pots). Tests were conducted twice. Susceptible plants developed typical symptoms of wilt as above, starting two to three weeks after inoculation, followed by plant death. Race 1 isolates gave either no or only very mild symptoms in the resistant cultivars. Control plants showed no symptoms. In cultivars where infection occurred, most plants developed typical symptoms. V. dahliae was recovered by plating from all plants with symptoms. Based on the differential reactions of the five cultivars, 10 isolates were classified as race 1 and 41 as race 2.

The appearance of V. dahliae race 2 in tomatoes in Tunisia poses a major threat to this crop, particularly in protected cultivation where no rotations are used and the absence of soil disinfection favours the build-up of soil-borne inoculum. V. dahliae race 2 has been reported from southern Europe, North and South America and North and South Africa (Pegg & Brady, 2002) but this is the first report from Tunisia.