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Evaluation of a Silk-Inoculation Technique to Differentiate Reactions of Sweet Corn Hybrids to Common Smut. J. K. Pataky, Department of Plant Pathology, University of Illinois, Urbana 61801; C. Nankam, and M. R. Kerns. Department of Plant Pathology, University of Illinois, Urbana 61801. Phytopathology 85:1323-1328. Accepted for publication 11 August 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-1323.

Reactions of 750 sweet corn hybrids to common smut, caused by Ustilago maydis, were evaluated in field trials using a silk-channel inoculation technique and natural infection. Hybrids were classified annually from resistant to susceptible based on incidence of galls on primary ears. Means and standard deviations of incidence of ear galls from plots inoculated with U. maydis were 34.2 20.9, 20 12.9, and 32.7 17.4% for the 1992, 1993, and 1994 trials, respectively. Means and standard deviations of incidence of ear galls from naturally infected plots were 1 2.4, 4.8 7.1, and 4.5 6.4% for the 1992, 1993, and 1994 trials, respectively. Hybrid means ranged from 0 to 96, 0 to 80, and 0 to 91% for inoculated plots and from 0 to 19, 0 to 47 and 0 to 58% for naturally infected plots in 1992, 1993, and 1994, respectively. Mean incidence of ear galls was greatest for the group of white sh2 hybrids and least for the white se hybrids. Average smut incidence also was higher for groups of early-maturing hybrids than for all other hybrids. The consistency of classifications was compared for hybrids common to trials in different years. The null hypothesis was retained for Chi-square tests of independence of hybrid classifications between two trials, indicating a lack of relationship between trials. Approximately 24% (91 of 383) of all hybrids common to two trials were placed in the same resistance category in both trials. Forty-two percent of the hybrids differed by one category. Inconsistent classifications, differences of two or three categories, occurred for about 34% (131 of 383) of the hybrids common to two trials. Although the silk-channel inoculation method resulted in a significantly higher incidence of ear galls than did natural infection, this technique and our categorization procedure resulted in greater inconsistency between trials than is acceptable. These procedures must be modified to accurately assess the response of hybrids to common smut and to breed for increased levels of smut resistance.