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First Report of Phytophthora cactorum on American Ginseng (Panax quinquefolius) in Bulgaria

American ginseng (Panax quinquefolius) is a recently introduced crop in Bulgaria. In autumn 2001, several 2-year-old plants from Stara Zagora County exhibited symptoms of wilting and dying. Laboratory analysis also revealed some browning of the ginseng root surface and discoloration of the vascular tissues. During later stages of the disease, roots became soft, rubbery, and disintegrated. After storage in a humid chamber for 3 to 5 days, roots were covered with a white, cottony mycelium. Following the transfer onto potato dextrose agar, this fungus formed rounded colonies of white, aerial mycelium. Pathogenicity of the isolate was demonstrated by inoculation of roots that were surface-disinfected with alcohol (70%) for 30 s and rinsed with sterile water. Roots were wounded with a scalpel, and agar pieces from a 1-week-old culture were placed under the cortical tissue. Five inoculated root pieces were kept in a humid chamber at 24 to 25°C, and the pathogen was reisolated subsequently from necrotic lesions that developed from wounds. No symptoms were found in the five wounded but noninoculated control roots. The pathogen was reisolated from the diseased tissue to fulfill Koch's postulates. Microscopic examination showed that the pathogen had an aseptate mycelium (mean diameter of 5.3 μm), did not form hyphal swellings or chlamydospores, and had simple sympodial branching of the sporangiophores. Sporangia had a caducous nature with a pedicel length of 4.7 μm (1.7 to 6.7 μm). Sporangia were ovoid to obpyriform in shape, papillate, and nonproliferating measuring 30.6 (26.6 to 40.0) μm × 24.3 (23.3 to 30.0) μm. The length/width ratio varied between 1.25 and 1.3. The fungus was homothallic and produced paragynous antheridia and spherical oogonia with a diameter of 30.6 μm (26.6 to 33.3 μm) on V8 agar and in petri solution. Oospores were aplerotic and spherical (25 to 30 μm in diameter). Based on symptoms and pathogen characteristics (2), the disease was identified as Phytophthora root rot caused by Phytophthora cactorum. Additionally, the identity of the isolate was verified by sequence determination of the ribosomal internal transcribed spacer I region and alignment to the GenBank-EMBL DNA database (1), which revealed 100% sequence similarity with P. cactorum. To our knowledge, this is the first report of P. cactorum on American ginseng in Bulgaria.

References: (1) S. F. Altschul et al. Nucleic Acids Res. 25:3389,1997. (2) D. C. Erwin and O. K. Ribeiro. Morphology and identification of Phytophthora species. Pages 96--125 in: Phytophthora Diseases Worldwide. The American Phytopathological Society, St. Paul, MN, 1996.