First Report of Pythium tracheiphilum Causing Wilt and Leaf Blight on Lettuce (Lactuca sativa) in Spain

Since 2002, a disease on lettuce plants (cv. Estibaliz) was observed in the field and in greenhouses in the Principality of Asturias in northern Spain. The disease was estimated to cause losses of 20% in field-grown crops during the summer. Plants with aboveground symptoms of wilt and necrosis and darkening of vessels and necrosis in the tap root were collected. On potato dextrose agar (PDA), a fungus was recovered and identified as Pythium tracheiphilum according to the following characters: terminal or intercalary, globose or subglobose sporangia; zoospores formed in water; chlamydospores present in old cultures; smooth, globose, terminal, and intercalary oogonia; and smooth, globose oospores (2). For pathogenicity tests, two isolates were each inoculated on 9 seedlings (3 true leaves) and 9 plants (6 to 8 true leaves) of lettuce (Batavia type) grown in peat. Inoculum was prepared by suspending mycelium of the isolates collected from two PDA plates in 100 ml of distilled water with 10 ml poured around each plant. Control plants received distilled water only. Plants were maintained at 21°C and 80% relative humidity with a 12-h photoperiod. Symptoms were observed after 4 or 7 days for seedlings and plants, respectively. Within 20 days, 33% of the seedlings and plants died, and all surviving plants showed symptoms. Symptoms did not occur on the control plants. P. tracheiphilum was reisolated from inoculated plants but not from the control plants. This fungus has been described previously as a lettuce pathogen in France and Italy (1) but to our knowledge, this is the first report of P. tracheiphilum infecting lettuce in Spain. Reference: (1) C. M. Messiaen et al. Enfermedades de las hortalizas, Mundi-prensa, Madrid. 1995. (2) A. J. van der Plaats-Niterink. Stud Mycol. 21:1, 1981.