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.

First report of Fusarium fujikuroi causing root rot and seedling elongation of soybean in Indiana.

In summer 2020, 127 soybean (Glycine max (L.) Merr) seedlings (V1-V3 stage) showing reduced vigor or crown lesions were collected at Purdue's Agronomy Center for Research and Education in West Lafayette, Indiana. Root tissues from two seedlings with necrotic cotyledons and root rot were surface-sterilized and plated on dichloran-chloramphenicol-peptone agar (Andrews and Pitt 1986). Emerging hyphal tips were transferred to potato dextrose agar (PDA). Single-spore cultures were obtained and grown on PDA. Both isolates developed floccose white aerial mycelia with reddish-pink coloration in the media in 2 weeks on the benchtop. On carnation leaf agar, macroconidia formed on orange sporodochia within 2 weeks in darkness at 25C. Macroconidia were 3-5 septate, measuring 26 - 41 × 2.5 - 3.7 µm (avg. 34.8 × 3.2 µm, n=40). Microconidia were abundant in chains and false heads forming on both mono- and polyphialides, and measured 2.5 - 8.75 x 2.5 µm (avg. 5.9 × 2.5 µm, n=40). These characteristics were consistent with species descriptions of F. fujikuroi [Sawada] Wollenw. (teleomorph Gibberella fujikuroi) (Leslie and Summerell 2006). DNA was extracted from mycelium and the following genes were amplified and sequenced: the internal transcriber spacer (ITS) region using ITS1/ITS4 primers (White et al. 1990) (GenBank accessions MW463362/MW463363), the mitochondrial small subunit (mtSSU) rDNA using MS1/MS2 primers (White et al. 1990) (MW465310/MW465307), and the partial translation elongation factor 1-alpha (TEF1α) gene using 983F/1567R primers (Rehner and Buckley 205) (MW475297/MW475298). In GenBank BLAST searches, these sequences showed 100% identity to both F. proliferatum and F. fujikuroi. Species-specific forward primers Fuji1F and Proli1F were then used in combination with reverse primer TEF1R to amplify another region in the TEF1α gene (Amatulli et al. 2012). Proli1F/TEF1R primers failed under a variety of annealing temperatures while Fuji1F/TEF1R primers succeeded, and the products were sequenced (MW475299/MW475300). GenBank BLAST searches revealed 100% identity of both isolates to F. fujikuroi (MT448248.1). A pathogenicity test was conducted with isolate AC13 in the greenhouse following the protocol of (Ellis et al. 2013). Ten seeds (cv. Williams) each were used for inoculation and control, respectively, with one seed per cup. Root rot symptoms similar to those observed in the field were observed 14 days after planting on all inoculated plants but not on controls (VC stage). Infected plants showed symptoms of pre-emergence damping off, reddish-brown lesions on the tap and lateral roots, and root necrosis. Three plants also exhibited hyper-elongation of the stem (12.5, 11.1, and 18 cm, vs controls: avg. 6.8 cm, max. 8.5 cm, stdev 0.78 cm). F. fujikuroi was successfully reisolated from inoculated plants but not from controls and identified as described above. F. fujikuroi has been reported causing soybean root rot in China (Zhao et al. 2020), Korea (Choi et al. 2019), and the state of Kansas (Pedrozo et al. 2015). To our knowledge this is the first report of F. fujikuroi infecting soybeans in the state of Indiana. F. fujikuroi is known to cause elongated seedlings in rice (Leslie and Summerell 2006). Pedrozo et al. (2015) reported that F. fujikuroi isolated from soybean caused seedling elongation in rice but not in soybean. The increased distribution and new host symptomology observed here warrants heightened attention for the control of this pathogen.

First report of Mycoleptodiscus terrestris causing root rot of soybean in Indiana.

During the summers in 2019 and 2020, 137 soybean (Glycine max (L.) Merr) seedlings (V1-V3 stage) showing stunting, delayed emergence, and/or crown lesions were collected at Purdue's Agronomy Center for Research and Education in West Lafayette, Indiana. Four seedlings were stunted with reddish-brown girdled lesions along the hypocotyl and crown, rotted tap and lateral roots, and brown discoloration of the cortex and vascular tissues. Four fungal isolates (AC4, AC58, AC96, and AC127) were recovered by plating surface-sterilized symptomatic root tissue onto water agar plates and incubating on the benchtop until mycelia emerged. The growing hyphal tips were transferred to the semi-selective medium DCPA (Andrews and Pitt 1986). On potato dextrose agar, the fungal colonies developed olivaceous green mycelia which melanized into a mat of black microsclerotia with time and no conidia were observed. On 1.5% water agar plates amended with twice autoclaved soybean leaf and root tissue collected from flowering soybean plants, conidia were formed in sporodochia in darkness at 28 οC within one week. Conidia were 1-2 septate, cylindrical with two setae on either end, and measured 20.8 to 26.4 x 4 to 5.6 µm (average 23.9 x 4.7 µm, n=20). The morphological characters matched with the description of Mycoleptodiscus terrestris (Gerd.) Ostaz (Gerdemann 1953). Species identification was further confirmed by sequencing the internal transcribed spacers (ITS) region of rDNA amplified by ITS1 and ITS4 primers (White et al. 1990) and the translation elongation factor 1 alpha (TEF1-α) gene using 983F and 1567R primers with annealing temperature at 53 ○C (Rehner and Buckley 2005). The sequences were deposited in GenBank under the following accession numbers: ITS: MW002684, MT998441, MW010258, and MW010260; and TEF1-α: MW015941-MW015944. The GenBank BLAST searches revealed 100% identity in the ITS region (accession NR_145373.1) and 99.75% identity in the TEF1-α region (MK495977.1) to M. terrestris. Pathogenicity test was conducted on soybean seedlings (cv. Williams) at V1 growth stage using a root dip assay. Isolate AC58 was grown in a modified cotton seed meal broth (CSMB) to produce microsclerotia as inoculum (Gray 1978; Shearer and Jackson 2006). Microsclerotia concentration was measured using a hemocytometer and adjusted to 1.5 x 104 per ml. Five soybean seedlings each were dipped into inoculum or sterile CSMB for 30 minutes then planted individually in vermiculite-filled Styrofoam cups placed on flooded trays in 16-hr photoperiod light racks at room temperature. Seven days after inoculation, all inoculated plants were visibly stunted with root and crown symptoms identical to field symptoms while all controls were healthy. M. terrestris was successfully re-isolated from inoculated plants, but not from the controls, and identified by morphology and sequencing as above. M. terrestris has been previously reported causing root rot of soybean in Illinois (Gray 1978) and Wisconsin (Smith et al. 1998). To our knowledge, this is the first report of M. terrestris infecting soybean in Indiana. Increased geographic distribution of this pathogen warrants more attention for its control. M. terrestris has been proposed as a biological control agent against multiple aquatic weeds (Verma and Charudattan 1993; Shearer and Jackson 2006). Introduction of this fungus into soybean production regions should be avoided.