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Investigation of new soil born pathogen on soybean (Glycine max) in Tennessee

Rachel Guyer: University of Tennessee

<div>In August 2017, foliar chlorosis and root necrosis symptoms were observed in a soybean (<em>Glycine max</em>) production field in Hardeman County, Tennessee. Diseased plants (13) were collected from the field, from which 7 unidentified fungal colonies were isolated on potato dextrose agar amended with chloramphenicol and streptomycin sulfate (PDA-CS). Two of the isolates recovered were confirmed using morphological and molecular phylogenetic methods to be closely related to the <em>Xylaria </em>sp. that is known to cause taproot decline. Koch’s postulates were performed for both isolates in a greenhouse trial using the soybean variety Asgrow 4632. Inoculum was grown on sterilized soybean stems for one month prior to inoculation. Three treatment groups (non-treated check, sterilized soybean stems, and inoculated soybean stems) were included with four replicates each. Plants were watered every 1-2 days and grown under supplemental lighting with a 16-hr light:8-hr dark cycle with temperatures ranging from 25-35°C. Foliar symptoms were observed in inoculated plants after 3-4 weeks, and the pathogen was re-isolated from infected roots. The experiment was repeated twice. Taproot decline is an emerging disease, previously reported in Alabama, Mississippi, Louisiana, and Missouri. This is the first report of taproot decline in Tennessee, a disease that has been experimentally shown to be economically significant to soybeans, one of the most important crops produced in Tennessee.</div>