First report of Fusarium commune causing root rot of soybean seedlings in Indiana

In the summer of 2020, 127 soybean [Glycine max (L.) Merr] seedlings (V1-V3 stage) with reduced growth vigor were sampled as part of a bulk collection of seedling pathogens from Purdue's Agronomy Center for Research and Education in West Lafayette, Indiana. After rinsing off soil, one plant displayed prominent necrotic lesions on both cotyledons and the hypocotyl and rot of the roots. Root tissue segments measuring roughly 5 mm in length and adjacent to lesions were excised and surface sterilized in 0.6% NaOCl for 10 min, then in 70% ethanol for 2 min, rinsed thrice in sterile distilled H2O, and plated on dichloran-chloramphenicol-peptone agar (Andrews and Pitt 1986). Single-spore cultures were produced and grown on potato dextrose agar. The isolate (AC101) developed white aerial mycelium, rings of magenta coloration in the media, and pale orange sporodochia with age. Microscopic observation of two-week-old cultures grown on synthetic low-nutrient agar (NRRL Medium No. 4) in the dark at 28°C revealed 2-3 septate falcate macroconidia measuring 17.1 - 43.9 × 2.8 - 3.5 µm (avg. 29.4 × 3.1 µm, n=20); 0-1 septate straight to slightly curved microconidia measuring 3.9 - 8.6 × 1.9 - 2.5 µm (avg. 7.0 × 2.2 µm, n=20); and round chlamydospores borne singly or doubly with diameter measuring 6.1 - 14.2 µm (avg. 8.9 µm, n=20). These characteristics were consistent with descriptions of Fusarium commune K. Skovg., O'Donnell & Nirenberg (Skovgaard et al. 2003). DNA was extracted from aerial mycelium and the internal transcribed spacer (ITS) region using ITS1/ITS4 primers (White et al. 1990) (GenBank accession MW463361), the mitochondrial small subunit (mtSSU) rDNA using MS1/MS2 primers (White et al. 1990) (MW466537), and the translation elongation factor 1-alpha (TEF1α) gene using 983F/1567R primers (Rehner and Buckley 2005) (MW475296) were amplified and sequenced. Blast searches in GenBank showed that these sequences had 100% identity with corresponding sequences of F. commune (ITS: MN452698; mtSSU: AF362277; and TEF1α: KU171720). The matching mtSSU sequence was an accession from the original species description (Skovgaard et al. 2003). A pathogenicity test was conducted under greenhouse conditions (20-29°C, avg. 24°C) following the infested soil protocol of Ellis et al. (2013a). Ten seeds (cv. Williams) each were used in inoculated and mock-inoculated control treatments with one seed per foam cup. Root rot symptoms similar to, but more destructive than those observed in the field, were observed 14 days after planting on all inoculated plants but not on controls. Inoculated plants reached VE stage compared to controls which reached VC. Disease symptoms included severe necrotic lesions on the cotyledons, dark brown rot of the developing tap root, and brown hypocotyl lesions similar to field symptoms. F. commune was successfully reisolated from inoculated plants, but not from controls, as described above. F. commune has been reported to cause soybean root rot in China (Chang et al. 2018), Korea (Choi et al. 2020), as well as Iowa (Ellis et al. 2013b). To our knowledge this is the first report of F. commune infecting soybean seedlings in the state of Indiana. The expanded distribution of this soybean pathogen warrants heightened attention for its control.