First Report of Exserohilum pedicellatum Causing Root Rot of Corn in Indiana

Corn (Zea mays) samples were submitted to the Purdue Plant and Pest Diagnostic Lab (PPDL) in 2018 (mature plants), 2019 (seedlings), and 2020 (seedlings) from multiple Indiana counties (DeKalb, Monroe, Jasper, Tippecanoe, and La Porte) that showed general stunting and moderate to severe root necrosis. Symptomatic roots were surface sterilized with 10% sodium hypochlorite for 1 min, rinsed with sterile water, and plated on quarter-strength potato dextrose agar (QPDA). A dematiaceous fungus that readily produced conidia was isolated from the affected roots of each sample. Conidia were straight or fusiform with a pedicel-like extension and prominent hilum, olive-brown to dark brown in color, measured 42 to 96 × 19 to 34 µm (mean 77.8 × 23.8 µm; n = 30), and had four to five distoseptations. The fungus was identified as Exserohilum pedicellatum (A.W. Henry) K.J. Leonard & Suggs based on morphological characteristics (Hernández-Restrepo et al. 2018; Sivanesan 1987). Isolate PPDL 19-00597, acquired from a seedling from Tippecanoe County received in June 2019, was submitted to the United States Department of Agriculture Agricultural Research Service Northern Regional Research Lab Culture Collection (NRRL no. 66965) and the Purdue Herbarium (PUL F27759). DNA was extracted from the single-spore isolate using the ZR Fungal/Bacterial DNA MiniPrep kit (Zymo Research, Irvine, CA) and amplified using ITS1 and ITS4 primers (White et al. 1990). The 492-bp fragment (GenBank accession MW376920) was 99 to 100% identical to the E. pedicellatum type specimen (CBS 322.64; GenBank NR_157425). To determine pathogenicity, corn seedlings (Burpee’s Triple Crown Bicolor Hybrid) grown for 1 week in sterile sand and washed free of sand were placed for 10 min in a beaker with a spore suspension made from 50 ml of sterile deionized water and conidia and hyphal fragments scraped from the agar surface of two QPDA plates containing 2 weeks of growth. Control plants were placed for 10 min in a beaker with only 50 ml of sterile deionized water. Three inoculated or noninoculated seedlings were planted in sterile sand in 0.5-liter plastic pots, for a total of three pots inoculated and three pots noninoculated plants. Plants were grown in a greenhouse at 25°C for 4 weeks. Pathogenicity testing was repeated once. Inoculated plants developed large gray-to-brown lesions on primary and seminal roots, whereas the control plants remained healthy. Isolations from asymptomatic plant roots produced no fungal growth, whereas the fungus was consistently reisolated from symptomatic plants and identified as E. pedicellatum by morphological characteristics. To the best of our knowledge this is the first report of this disease on corn in Indiana. The pathogen has been found causing disease on corn in Australia, South Africa, and in eight other states in the United States, and although it has not been identified as a major pathogen of agronomic crops in the North Central Region of the United States at this time (Farr and Rossman 2021; Gilbert 2003; Isakeit et al. 2007; NPDN-NDR 2021), it has been observed in increasing frequency and may contribute to localized yield loss.