Occurrences of Diplodia Leaf Streak Caused by Stenocarpella macrospora on Corn (Zea mays) in Illinois

Abstract

In August 2008, long and narrow lesions were observed on leaves of corn (Zea mays L.) growing in a field in Pope County, Illinois. Lesions were 10 to 35 × 50 to 250 mm and were cream to tan. Dark pycnidia inside the lesions were immersed and approximately 350 μm in diameter. Affected leaves were collected and placed into a moist chamber to encourage the development of conidia. Conidia developed in cirri and were dark, one septate, and 7 to 11 × 59 to 87 μm. Cirri were streaked onto potato dextrose agar (PDA; Becton, Dickinson, and Company, Franklin Lakes, NJ) and cultures arising from single conidia were transferred and maintained. On the basis of the corn leaf symptoms and the morphological characteristics of the pycnidia and conidia, the fungus was tentatively identified as Stenocarpella macrospora (Earle) Sutton (1). To complete Koch's postulates, ‘Garst 84H80-3000GT’ corn was inoculated in the greenhouse. Conidia were produced by placing a S. macrospora isolate from Pope County, IL onto water agar containing autoclaved corn leaves and incubating at room temperature until pycnidia and conidia were produced (approximately 3 weeks). A conidial suspension was used to inoculate the leaf whorls of corn plants (approximately at the V4 growth stage). Control plants were mock inoculated with sterile water. The experiment was repeated once over time. Twenty days after inoculation, all plants inoculated with S. macrospora conidia developed lesions similar to those observed in the field, and mock-inoculated plants remained symptomless. The fungus was reisolated on PDA from the symptomatic leaves. In August 2009, symptomatic leaves similar to those observed in Pope County, IL in 2008 were observed and collected from corn fields in Gallatin and Vermillion counties. Pycnidia and conidia from these lesions were similar to those described above, and isolates from single conidia were obtained from these samples. To confirm the identity of all isolates collected, PCR amplification of the small subunit rDNA and internal transcribed spacer (ITS) region with primers EF3RCNL and ITS4 was conducted (3). The PCR product was sequenced with these primers at the Keck Biotechnology Center at the University of Illinois, Urbana. The resulting nucleotide sequence was compared with small subunit rDNA and ITS sequences deposited in the GenBank nucleotide database, which revealed 99% homology to sequences of S. macrospora. In total, six of our S. macrospora isolates from Gallatin, Pope, and Vermillion counties were submitted to the United States Department of Agriculture–Agriculture Research Service Culture Collection in Peoria, IL, where they have received NRRL Accession Nos. 54190–54195. To our knowledge, this is the first report of S. macrospora affecting corn in Illinois. Although not observed in the Illinois corn fields described above, S. macrospora has been reported to infect stalks and ears (2). Because of the large leaf lesions caused by S. macrospora and its reported aggressiveness in causing disease on leaves, ears, and stalks, this pathogen has the potential to cause severe yield and quality losses to corn in the United States (2). References: (1) M. L. Carson. Diseases of minor importance or limited occurrence. Page 23 in: Compendium of Corn Diseases. 3rd ed. The American Phytopathological Society, St. Paul, MN, 1999. (2) F. M. Latterell and A. E. Rossi. Plant Dis. 67:725, 1983. (3) N. S. Lord et al. FEMS Microbiol. Ecol. 42:327, 2002.