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First Report of Black Root Rot Caused by Thielaviopsis basicola on Soybean (Glycine max) in Arkansas

Thielaviopsis basicola (Berk. & Broome) Ferraris (synonym Chalara elegans Nag Raj & Kendrick) is a soilborne plant-pathogenic fungus reported in many parts of the world. In Arkansas, T. basicola is found commonly in cotton fields (4). This fungus colonizes cortical tissue of seedlings under cool wet conditions, causing a dark brown or black discoloration of the roots and hypocotyls, resulting in stunted, slow-developing plants (4). In 2008, large areas of stunted soybean plants with shortened internodes were reported in a field in Phillips County, AR, where cotton had previously been produced. Soybean was planted in this field in early April when cool soil temperatures (~21 to 24°C) prevailed. Soybean plants at the v3 to v5 growth stages were observed to have extensive areas of black cortical root necrosis. Plant samples were collected and roots were excised, washed, and surface disinfested in a 10% NaOCl solution. Root segments were incubated on the carrot-based selective medium TB-CEN (3). T. basicola was isolated from incubated segments after 2 weeks at 21°C in the dark. Chlamydospore chains (44.8 to 56.0 × 8.4 to 11.2 μm) consisting of an average of six spores and endoconidia (8 to 30 × 3 to 5 μm) were observed with a compound microscope. In addition to plant tissue, soil was assayed and confirmed to be positive for T. basicola by the pour plate technique (3) with the medium TB-CEN. Greenhouse trials were conducted to confirm field observations. Soil from the Phillips County field was sterilized and reinfested with 100 CFU of chlaymdospore suspension per gram (dry weight) of soil. Fifty soybean seeds (cv. Schillinger 457) were planted in infested and sterilized soil and grown for 29 days. Results showed that 38% of plants germinated and survived in the T. basicola-infested soil compared with 71% in the sterile soil treatment. Fifteen of the nineteen plants that survived in the infested soil were positive for T. basicola, while all plants in the sterilized soil were negative for the fungus. Soybean has previously been reported to be a host of T. basicola worldwide, but North American reports have been confined to Canada and Michigan, where cool soil temperatures persist for longer periods during the early part of the growing season (1,2). To our knowledge, this is the first report of T. basicola being important in the growth of soybean in warmer latitudes where the pathogen has been observed frequently on cotton and tobacco. In areas where cotton has historically suffered seedling damage from T. basicola, black root rot may become important on soybean as production of the latter crop increases. Since the initial field observation and confirmation in 2008, multiple soybean fields in 10 Arkansas counties have been documented with black root rot, with an estimated 5 to 30% of plants in each field infected.

References: (1) T. R. Anderson. Can. J. Plant Pathol. 6:71, 1984. (2) J. L. Lockwood et al. Plant Dis. Rep. 54:849, 1970. (3) L. P. Specht and G. J. Griffin. Can. J. Plant Pathol. 7:438, 1985. (4) N. R. Walker et al. Phytopathology 89:613, 1999.