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First Report of Root Rot Incited by Thielaviopsis basicola on Lamb's Lettuce (Valerianella olitoria) in Europe

Lamb's lettuce (Valerianella olitoria) is increasingly grown in Italy and used in the preparation of processed salad. In the fall of 2003, plants of lamb's lettuce cvs. Trophy and Palmares showing symptoms of severe stunting were observed in several commercial plastic greenhouses near Bergamo in northern Italy. The distribution of the disease was generally uniform in the greenhouses and 10 to 30% of the plants were affected. The first symptoms, consisting of reduced growth of the plants and extensive chlorosis, developed in October at temperatures ranging between 10 and 20°C on 30-day-old plants. The roots of diseased plants showed extensive necrosis that extended to the crown of the plants. The diseased tissue was disinfested for 1 min in 1% NaOCl and plated on potato dextrose agar amended with 100 µg/l streptomycin sulfate. A fungus with the morphological characteristics of Thielaviopsis basicola was consistently and readily isolated from symptomatic tissues (1). Catenulate, cylindrical, hyaline endoconidia and catenulate, subrectangular, thick-walled chlamydospores (aleuriospores) were observed. Ten-day-old plants of cvs. Trophy and Palmares were artificially inoculated by dipping three isolates of T. basicola obtained from infected plants for 15 min in a spore suspension (1 × 10⁶ CFU/ml). Noninoculated plants served as control treatments. Each treatment consisted of 30 plants. Plants were maintained at 20°C in a growth chamber, with 12 h of light/day. Symptoms developed 25 days after the artificial inoculation on both cultivars, and T. basicola was consistently reisolated from diseased plants. The noninoculated plants remained healthy. The pathogenicity test was carried out twice. To our knowledge, this is the first report of T. basicola on lamb's lettuce in Italy as well as in the world. The same disease was described on corn salad (Valerianella locusta) in the United States (2).

References: (1) D. E. Mathre and A.V. Ravenscroft. Phytopathology 56:337, 1966. (2) M. E. Stanghellini et al. Plant Dis. 74:81, 1990.