First report of Verticillium albo-atrum causing wilt in Ve-resistant tomato in the United Kingdom

In 1996, Verticillium albo-atrum was isolated from wilting tomato plants of a cultivar reportedly resistant to the fungus, from a hydroponic crop in Kent, UK. Plants had small leaves, reduced vigour, and pale-brown discoloration of stem vascular tissue extending up to 3 m above the base. Wilting occurred from the upper leaves downward, becoming more common as the disease progressed; eventually, but not always, plants died. On severely wilted plants, grey stem lesions bearing abundant sporulation of V. albo-atrum were occasionally observed.

Wilt caused by V. albo-atrum has subsequently been confirmed on other nurseries in the UK in several resistant cultivars (e.g. Cloe, Espero, Ferrari). Resistance to V. albo-atrum is conferred by a single dominant gene (Ve) (Schaible et al., 1951) that has remained effective for over 20 years. Tests were therefore undertaken to compare pathogenicity of V. albo-atrum recently isolated from Ve-resistant tomato cv. Espero (AR01/036) with a control isolate (T179) obtained from tomato in 1974, before widespread introduction of Ve-resistance into European cultivars.

At the two-leaf stage, roots of seedlings of Ve-susceptible (Shirley) and resistant (Espero) cultivars (Fletcher, 1992; Pinetree de Ruiter Seeds, Southampton, UK) were placed for 10 min in a conidial suspension (5 x 10⁶ ml^-1) of V. albo-atrum in de-ionised water, or in water alone, before transplanting into pots. Twenty plants per treatment were placed in a glasshouse maintained at c. 17°C. Most plants of both cultivars inoculated with isolate AR01/036 showed symptoms 4 weeks after inoculation, while isolate T179 caused symptoms in cv. Shirley only. At 5 weeks after inoculation, the mean % leaf area affected in cvs Shirley and Espero, respectively, was 14.8 and 11.4% with isolate AR01/036 and 8.3 and 0.1% with the control isolate. Uninoculated plants showed no symptoms of wilt. Verticillium albo-atrum was recovered from stained stem vascular tissue of inoculated plants.

Isolate AR01/036 of V. albo-atrum is clearly pathogenic to Ve-resistant tomato. A strain of V. albo-atrum aggressive on Verticillium-resistant tomato has previously been reported from the Netherlands (Paternotte & Van Kesteren, 1993). This is the first report of a strain of V. albo-atrum causing wilt in crops of Ve-resistant tomatoes in the UK.