Extensive blighting associated with root rot was seen in a 0.5-ha field of flowering corn (Zea mays inbred line ‘Tx714B’) at the Texas Agricultural Experiment Station in Weslaco, TX (Hidalgo County) in October of 2006. All plants were affected and most of the leaves of each plant were dead. Plants subsequently failed to produce any seed. Primary and secondary roots of blighted plants had extensive, dark brown necrotic lesions. Symptomatic root samples from 11 plants were surface disinfested with 10% bleach for 1 min, followed by two rinses with sterile distilled water, and plated onto water agar plus streptomycin. Fungal growth was transferred to one-fifth-strength potato dextrose agar plus streptomycin (PDAS). A dematiaceous hyphomycete was isolated from 45% of plated root segments from eight plants. The isolates had characteristics consistent with Exserohilum pedicellatum (Henry) Leonard & Suggs: straight, elongated conidia 57 to 75 μm long × 23 to 25 μm wide, with 4 to 6 pseudosepta, a protuberant hilum, and a prominent pedicel-like extension at the base (2). Four isolates were evaluated for pathogenicity. Conidia were scraped from PDAS plates, suspended in 100 ml of water, and mixed with 2.9 kg of autoclaved 1:1 (v/v) sand/sandy loam soil to obtain a density of 150 to 475 conidia per g of soil. Soil was placed in 20- × 4-cm-diameter plastic Cone-Tainer cells (Stuewe & Sons, Corvallis, OR) and a corn seed (‘Tx714B’) was planted 1.5 cm deep in each. Seeds were previously disinfested 2 min with 10% commercial NaOCl. Plants were kept in a growth chamber at 26°C for 4 weeks. There were six replicates per isolate and the experiment was repeated once. All four isolates caused a dark brown root necrosis covering 16 to 29% of the nodal roots and 43 to 83% of the seminal roots. Necrosis did not occur on roots from noninfested control containers. The pathogen was consistently reisolated from symptomatic roots, fulfilling Koch's postulates. This fungus is also pathogenic to sorghum (1), which is grown in rotation with corn in this field, increasing the chances that the fungus will persist. To our knowledge, this is the first report of the disease on corn in Texas.
References: (1) R. J. Shepherd et al. Phytopathology 57:52, 1967. (2) A. Sivanesan. IMI Descriptions of Fungi and Bacteria 89(887):1, 1986.
