First report of Aerial blight of soybean caused by Rhizoctonia solani AG1-IA in Tennessee

Rhizoctonia solani is the causal agent of aerial blight (AB) in soybean and rice, and banded sheath blight in corn and sorghum. Across the United States, yield loss caused by AB of soybean was estimated at 2.7 million bushels in 2023 (Crop Protection Network, 2024). During the first week of September 2021, soybean plants with blight symptoms were observed at the University of Tennessee AgResearch Education Center in Milan, TN. Diseased plants were predominantly found in the lower area of the field. Although symptomatic plants represented less than 1% of the entire 3.25-hectare field, disease severity ranged from 20% to 80%po. The field has been in continuous, no-till soybean production for over 15 years, planted on 76.2 cm centers. Symptoms included water-soaked to dead, matted soybean leaf tissue with mycelial growth in lower canopies. Infected soybean pods displayed the presence of dark sclerotia. Symptomatic leaves and sclerotia were isolated on potato dextrose agar (PDA) at 25°C and produced light brown colonies with sclerotia developing after five days. Colonies exhibited long hyphae (Ø = 13.4 mm, n = 100) with branches at near 90-degree angles, constrictions at branch points, and separation right after the branch. These characteristics resembled descriptions of Rhizoctonia solani (Atkins 1954; Rodriguez-Herrera et al. 2023). DNA was extracted from mycelia grown in potato dextrose broth, amplified, and sequenced targeting the internal transcribed spacer (ITS) region using primers ITS4 and ITS5 (White 1990) and the glyceraldehyde-3-phosphate dehydrogenase gene (GAPDH) using primers GAPDH F-5 and GAPDH R-5 (Bahadur et al. 2024). The consensus sequences were deposited in GenBank (accession nos. PV548822 and PV550786, respectively). A BLASTn search of NCBI GenBank revealed 98.14% identity of the 697bp for the ITS gene with R. solani (GenBank accession nos. MK481078.1) and 98.96% identity of the 768bp for the GAPDH gene with R. solani (GenBank accession nos. HQ425709.1). Fungus AG type 1-IA was identified using specific primers targeting the large subunit 28S region of the nuclear ribosomal DNA (Matsumoto 2002). Koch's postulates were fulfilled by inoculating soybean plants at the V1 stage with R. solani infested toothpicks. Toothpicks were colonized with the fungus on PDA plates for four days, then air-dried for one day before inoculation. The plants were grown in a greenhouse and inoculated by inserting the toothpicks into the main stem. The inoculated plants were then placed inside a plastic humidity chamber with misting for three minutes every hour at a temperature of 25-28°C under a light period of 14 hours per day. Twenty-one days after inoculation, plants developed blight symptoms similar to those observed in the field. Symptomatic tissues were surface-sterilized and placed on PDA, reproducing colonies consistent with R. solani. Non-inoculated plants were used as controls and remained asymptomatic. Although soybean aerial blight has been reported in rice-producing states around Tennessee, to our knowledge, this is the first official report of the disease in Tennessee. The pathogen was observed in a field where conditions typically do not favor its occurrence, such as not rotating with rice and having wide row spacing. A shift in pathogen behavior and disease development may pose a potential threat to soybean, corn, and grain sorghum production. This report serves as a reference for future studies on the distribution and habits of the pathogen.