In 2005 and 2006, field-grown iceberg lettuce (Lactuca sativa) in California's coastal Salinas Valley (Monterey County) was affected by a previously unreported disease. Symptoms were observed on iceberg lettuce at the post-thin rosette stage (8 to 12 leaves). Plants were stunted and slightly chlorotic. Fine feeder roots had numerous, small (4 to 8 mm long), elongated, dark brown-to-black lesions. Larger secondary roots and taproots lacked lesions. No vascular discoloration was present. Isolations from root lesions consistently resulted in gray fungal colonies that formed catenulate, cylindrical, thin-walled, hyaline endoconidia and catenulate, subrectangular, thick-walled, dark aleuriospores. The fungus was identified as Thielaviopsis basicola (2). Conidial suspensions (5.0 × 105) of eight isolates from iceberg lettuce were used for pathogenicity tests. Iceberg cv. Ponderosa and romaine cv. Winchester were grown for 3 weeks in soilless peat moss rooting mix. Roots of 20 plants per cultivar were washed free of the rooting mix and soaked in conidial suspensions for 5 min. Plants were repotted and grown in a greenhouse. Control plant roots were soaked in sterile distilled water (SDW). After 3 weeks, inoculated iceberg exhibited slight chlorosis in comparison with control plants. Feeder roots of all iceberg plants inoculated with the eight isolates exhibited numerous black lesions and T. basicola was reisolated from these roots. Romaine lettuce, however, did not show any foliar symptoms. Small segments of roots had tan-to-light brown discoloration and T. basicola was occasionally reisolated (approximately 40% recovery). Roots of control iceberg and romaine showed no symptoms. Results were similar when this experiment was repeated. To explore the host range of T. basicola recovered from lettuce, two isolates were prepared and inoculated as described above onto 12 plants each of the following: iceberg lettuce (cv. Ponderosa), bean (cv. Blue Lake), broccoli (cv. Patriot), carrot (cv. Long Imperator #58), celery (cv. Conquistador), cotton (cv. Phy-72 Acala), cucumber (cv. Marketmore 76), green bunching onion (cv. Evergreen Bunching), parsley (cv. Moss Curled), pepper (cv. California Wonder 300 TMR), radish (cv. Champion), spinach (cvs. Bolero and Bossanova), and tomato (cv. Beefsteak). Control plant roots of all cultivars were soaked in SDW. After 4 weeks, only lettuce and bean roots had extensive brown-to-black lesions, from which the pathogen was consistently resiolated. Roots of cotton, pepper, spinach, and tomato had sections of light brown-to-orange discoloration; the pathogen was not consistently recovered from these sections. All other species and the control plants were symptomless. This experiment was repeated with similar results except that inoculated peppers were distinctly stunted compared with control plants. To my knowledge, this is the first report of black root rot caused by T. basicola on lettuce in California. Disease was limited to patches along edges of iceberg lettuce fields; disease incidence in these discrete patches reached as high as 35%. Affected plants continued to grow but remained stunted in relation to unaffected plants and were not harvested. Black root rot of lettuce has been reported in Australia (1); that report also showed that lettuce cultivars vary in susceptibility to T. basicola and isolates from lettuce were highly aggressive on bean but not on many other reported hosts of this pathogen.
References: (1) R. G. O'Brien and R. D. Davis. Australas. Plant Pathol. 23:106, 1994. (2) C. V. Subramanian. No. 170 in: Descriptions of Pathogenic Fungi and Bacteria. CMI, Kew, Surrey, UK, 1968.
