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First Report of Binucleate Rhizoctonia AG-G on Common Turmeric (Curcuma longa) in the United States

Hall, M., Lawrence, K., Shannon, D., Gonzalez, T., Newman, M.
Plant disease 2019 v.103 no.4 pp. 771
Curcuma longa, DNA, DNA primers, National Center for Biotechnology Information, Rhizoctonia, ambient temperature, color, culture media, denaturation, ethanol, financial economics, genomics, germination, greenhouses, growers, internal transcribed spacers, medicinal plants, mycelium, pasteurization, pathogenicity, photoperiod, polymerase chain reaction, polystyrenes, rhizomes, sand, slurries, sodium hypochlorite, soil, turmeric, Alabama, India
Common turmeric (Curcuma longa L.) is a spice crop native to India (Prasad and Aggarwal 2011) and is cultivated as a part of the Auburn University Medicinal Plant Collection. During harvest in 2015, rhizomes showed symptoms of rhizome rot including loss of turgidity and a dark red-brown hue compared with the typical vibrant orange color. Symptomatic rhizomes were sliced into 0.1-cm cross sections and surface sterilized using 95% ethanol for 30 s followed by 0.625% NaOCl for 2 min and aseptically transferred to acidified potato dextrose agar (APDA) plates. Cultures were grown at ambient temperature (23°C) with 8 h of light for 3 to 10 days. Cultures were examined at 200 to 400× under an inverted microscope. Isolates exhibited typical morphology described for binucleate Rhizoctonia taxa, exhibiting brown elongated mycelial cells with near right-angle branching and sclerotium-like structures of 1 to 2 mm diameter (Roberts 1999). Two internal transcribed spacer regions of DNA were amplified using the primers ITS-1 and ITS-4 provided by Eurofins Genomics (Louisville, KY). The DNA thermocycler amplification procedure was initial denaturation at 94°C for 5 min followed by 35 cycles of 94°C for 1 min, 55°C for 1 min, and 72°C for 2 min. After amplification, polymerase chain reaction products were sequenced by Eurofins Genomics. The sequences for nine isolates were analyzed using the National Center for Biotechnology Information BLAST and shared 100% identity with binucleate Rhizoctonia AG-G (accession JF519837). These sequences were submitted to GenBank and can be accessed as accessions MG735735 to MG735752. Koch’s postulates were conducted to affirm the pathogenicity of the isolate on C. longa. The binucleate Rhizoctonia AG-G cultures were increased by placing 3-mm-diameter mycelial plugs from APDA plates into flasks containing 200 ml of potato dextrose broth. After 5 days of growth, the mycelium was blended into a slurry. C. longa rhizomes were sown in 500-cm³ polystyrene pots in a pasteurized soil/sand mix (1:1 v/v) housed in the greenhouse. Greenhouse temperature ranged from 22 to 34°C, and light was supplemented with 1,000-W halide bulbs to maintain the day length of 14 h. The 0.4% v/v binucleate Rhizoctonia AG-G slurry was pipetted onto 10 C. longa plants 14 days after germination. The experiment was terminated 60 days after inoculation and repeated once. Upon termination, rhizomes from all 10 inoculated plants exhibited symptoms of rhizome rot including disintegrating tissue and dark coloration. All control inoculated plants were healthy and revealed no disease symptoms. Rhizomes were sliced into 0.1-cm cross sections, surface sterilized, plated on APDA, and grown for 5 days. Binucleate Rhizoctonia AG-G was reisolated from all inoculated plants, thereby confirming Koch’s postulates. To our knowledge, this is the first report of binucleate Rhizoctonia AG-G infecting C. longa in the United States. Because the Rhizoctonia complex is endemic to Alabama soils (Palmateer et al. 2004), potential growers of turmeric should consider soilborne-fungus management strategies to minimize economic losses owing to rhizome rot.