Root Rot of Hydroponically Grown Lettuce Caused by Phytophthora drechsleri in Mexico

During March of 2008, bibb lettuce (Lactuca sativa L.) plants with severe wilting and root rot were observed in a commercial liquid-hydroponic greenhouse in Guanajuato, Mexico. By July of that year, the disease affected most plants in the facility. A Phytophthora sp. was consistently isolated from diseased roots on potato carrot agar. Several Phytophthora isolates were morphologically characterized. Sporulation was achieved by placing colonized disks of clarified V8 juice agar (V8A) into nonautoclaved soil extract (10 g avocado soil/1,000 ml distilled water, stirred for 3 h, and filtered). Sporangia were persistent, nonpapillate, and 40 to 58 μm long × 30 to 40 μm wide. External and internal proliferation was observed. Hyphal swellings were predominantly rounded. Oospores were not observed. The isolates grew on V8A at 35°C. Pathogenicity tests were conducted twice by utilizing a representative isolate (AC1) on bibb lettuce seedlings (10 replicates per experiment). Seeds were placed on sterile, water-soaked paper in petri dishes. After 10 days, each lettuce seedling was placed into a tube containing approximately 2 ml of sterile distilled water and 2,000 zoospores. Control plants were placed in tubes with water only. Plants were incubated for 7 days in a moist chamber at 25°C. Symptoms of wilting and root necrosis were observed 2 to 3 days after inoculation. All plants were dead 5 to 7 days after inoculation. A Phytophthora sp. was always isolated from the roots of inoculated plants. Control plants remained healthy. The pathogen was identified as Phytophthora drechsleri Tucker according to morphological characteristics. To confirm the identity of the pathogen, sequences of the internal transcribed spacers (ITS) were obtained from three representative isolates. The ITS sequences that were obtained shared 100% homology to several strains of P. dreschleri, including isolates from cucurbits (GenBank Accession No. AF228097). The ITS sequence was deposited in NCBI as Accession No. FJ790770. P. cryptogea and P. dreschleri have been reported as causing root rot on lettuce grown hydroponically in the United States and Korea (1,2). To our knowledge, this is the first report of P. drechsleri causing root rot on lettuce in Mexico. References: (1) H. J. Jee et al. Plant Pathol. J. 17:311, 2001. (2) A. R. Linde et al. Plant Dis. 74:1037, 1990.