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

Authors: DETRANALTES, CHRISTOPHER - Purdue University; Cai, Guohong

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2020
Publication Date: 2/24/2021
Citation: Detranaltes, C., Cai, G. 2021. First report of Mycoleptodiscus terrestris causing root rot of soybean in Indiana. Plant Disease. 105:1194. https://doi.org/10.1094/PDIS-09-20-2023-PDN.
DOI: https://doi.org/10.1094/PDIS-09-20-2023-PDN

Interpretive Summary: Soybean is an important crop. Numerous pathogens cause diseases on soybean and affect soybean yield and quality. The geographical range of pathogens changes constantly. Here we report a new pathogen, Mycoleptodiscus terrestris, causing root rot of soybean in Indiana. We completed the experiments confirming its pathogenicity. This pathogen has been previously reported causing soybean root rot in Illinois and Wisconsin. Knowledge of the expanded geographical distribution of this pathogen will provide insight in disease control.

Technical Abstract: During the summers of 2019 and 2020, symptomatic soybean (Glycine max (L.) Merr) seedlings (V1-V3 stage) were collected from Purdue’s Agronomy Center for Research and Education in West Lafayette, Indiana. One seedling from 2019 and three seedlings from 2020 were stunted and showed reddish-brown girdled lesions along the hypocotyl and crown. Upon removal from the soil, taproots and lateral roots were visibly necrotic and rotted. Cross sections of roots revealed brown discoloration of both the cortex and vascular tissues. Fungal isolates were recovered from these seedlings by plating surface-sterilized symptomatic root tissue on water agar plates and growing hyphal tips were transferred to DCPA semi-selective media plates (15 g peptone, 0.5 g KH2PO4, 0.5 g MgSO4.7H2O, 20 g agar, amended with 2 µg dichloran and 200 µg chloramphenicol, per liter) (Andrews and Pitt 1986). On potato dextrose agar, the fungal colonies developed olivaceous green mycelia which melanized with time but no conidia. Within a week at 28°C, mycelia had formed a dense crust of black microsclerotia on the surface of the plates. Conidia formed in dark brown fruiting structures on twice autoclaved soybean leaf tissue and roots amended onto 1.5% water agar plates. 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). These morphological characters matched the description of Mycoleptodiscus terrestris (Gerd.) Ostaz (Gerdemann 1953). Species identification was confirmed by sequencing ITS1 and ITS2 regions amplified by ITS1 and ITS4 primers (White et al. 1990) and the tef1-a region using 983F and 1567R primers (Rehner and Buckley 2005). The sequences were deposited in GenBank under the following accession numbers: ITS: MW002684, MT998441, MW010258, and MW010260, and tef1-a : MW015941-MW015944. BLAST searches revealed 100% identity in the ITS region (accession NR_145373.1) and 99.75% identity with tef1-a region (MK495977.1) with M. terrestris. Pathogenicity was tested on soybean seedlings (cv. Williams) using a root dip method with inoculum grown on a modified cotton seed meal broth (Gray 1978; Shearer and Jackson 2006). Root symptoms identical to field symptoms were observed on all inoculated plants but not on controls. The pathogen significantly reduced fresh root weight and seedling height. M. terrestris was successfully re-isolated from symptomatic tissue from inoculated plants, but not from the controls. Pathogenicity was also tested on soybean seeds using surface-sterilized soybean in a petri dish assay with 3-day old cultures grown on 2% water agar at 28 °C in darkness. There was no seed damping off. Germinated seedlings were stunted with dark brown lesions along the taproot. 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 the state of Indiana.