In spring 2012, maize farmers in southeast and south central Iowa reported stand losses due to pre- and post-emergence damping-off, and many of the fields had to be replanted. Symptoms of the disease included rotted seed, or brown, rotted, water-soaked mesocotyls and root tips. Maize seedlings with severe root and mesocotyl symptoms were yellow and wilted, stunted, or dead. The disease occurred approximately 2 weeks after cool, wet conditions. Symptomatic mesocotyls and roots were washed for 30 min, rinsed with sterile distilled water, and blotted dry on sterile paper towels. Isolation of the pathogen was performed by aseptically cutting 2- to 3-mm sections of tissue from the edge of a lesion, placing the segments under corn meal agar (CMA) containing pimaricin, ampicillin, rifampicin, and pentachloronitrobenzene (PARP), and incubating at 22°C in the dark. Colonies that developed were putatively identified as Pythium species based on morphological characteristics and cultural features when compared to published descriptions (2,3). Characteristics of isolate IAC12F21-3 included spherical and smooth-walled oogonia 18 to 26 μm in diameter, monoclinous or usually diclinous antheridia 10 to 22 μm long and 5 to 10 μm wide with one or occasionally two per oogonium, and plerotic oospores 15 to 25 μm in diameter. Sporangia were globose to ellipsoidal, 22 to 41 μm in diameter, and zoospores were 7 to 10 μm long. Primers ITS1 and ITS4 were used to amplify the ITS region within clade E1 of 88 isolates. The resultant amplicons were sequenced and a BLAST search in GenBank confirmed isolate IAC12F21-3 as Pythium schmitthenneri based on 100% similarity with GenBank accession numbers JF836869 and JF836870. Pathogenicity testing was conducted using seed and seedling assays (1,4). Koch's postulates was performed by sampling pieces of symptomatic mesocotyl and root tissue from the inoculated pots, placing segments under CMA + PARP, and incubating at 22°C. Symptoms were similar to those observed in the field and P. schmitthenneri was re-isolated successfully. Non-inoculated control plants showed no symptoms. This is the first report of P. schmitthenneri causing seedling blight on maize in Iowa. Previously, P. schmitthenneri was reported as a pathogen on maize in Ohio (2).
References: (1) K. Broders et al. Plant Dis. 91:727, 2007. (2) M. Ellis et al. Mycologia, 104:477, 2012. (3) J. Middleton. Memoirs of the Torrey Botanical Club 20:171, 1943. (4) A. Rojas et al. Phytopathology, 102(Suppl):S5.8, 2012.
