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Incidence of Bipolaris and Exserohilum Species in Corn Leaves in North Carolina. K. J. Leonard, Research Plant Pathologist, USDA-ARS, Department of Plant Pathology, Box 7616, North Carolina State University, Raleigh 27695-7617. R. P. Thakur, and S. Leath. Plant Pathologist, Pearl Millet Improvement Program, ICRISAT, Patancheru P.O., Andhra Pradesh 502324, India; and Research Plant Pathologist, USDA-ARS, Department of Plant Pathology, Box 7616, North Carolina State University, Raleigh 27695-7616. Plant Dis. 72:1034-1038. Accepted for publication 12 July 1988. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1988. DOI: 10.1094/PD-72-1034.

Samples of 4260 corn leaves were collected without regard to disease symptoms at 2-m intervals in each of eight cornfields in western and two in eastern North Carolina in 1985. Sampling dates for the 10 fields ranged from 16 July to 8 August. The leaves were surface-sterilized and incubated in moist chambers for isolation of large-spored species of Bipolaris and Exserohilum. Incidence of B. zeicola infection was high (4181%) in leaves from nine of the 10 fields. B. maydis and E. turcicum, which are more aggressive pathogens of hybrid corn than B. zeicola, occurred at lower incidences of 048% and 050%, respectively, in leaves from these fields at the time of sampling. This indicated that initial population densities of these aggressive pathogens were much lower than that of B. zeicola. Forty-five isolates of E. turcicum collected before 28 August were all race 1. Three isolates of race 2 were obtained later from fields of hybrids with the Ht 1 gene for resistance to race 1. All 75 isolates of B. maydis obtained from the 10 extensively sampled fields were race O; they were equally virulent on inbred line B73 with normal cytoplasm or B73 with C, S, or T male sterile cytoplasm. Frequencies of mating types of B. maydis and B. zeicola varied from field to field, with no correlation between the frequencies in the two species. Races 2 and 3 of B. zeicola, however, had similar mating type frequencies in these fields. B. sorokiniana was isolated from leaves from six of the 10 fields; the greatest incidence was 37%. E. rostratum was found in leaves from five fields at incidences up to 18%, and E. holmii occurred in leaves in one field at 11% incidence. These data indicate that a variety of species of Bipolaris and Exserohilum are able to infect green leaves of mature corn plants in the field. Thus, corn may contribute to the survival of species that are primarily pathogenic on other gramineous hosts.

Keyword(s): Cochliobolus carbonum, C. heterostrophus, maize, northern leaf blight, southern leaf blight, Zea mays.