Classification of Sweet Corn Hybrid Reactions to Common Rust, Northern Leaf Blight, Stewart's Wilt, and Goss' Wilt and Associated Yield Reductions

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Abstract
Seventy-five, 120, and 100 commercial sweet corn [Zea mays] hybrids were evaluated in 1984, 1985, and 1986, respectively, for reactions to inoculations with Puccinia sorghi, Exserohilum turcicum, Erwinia stewartii, and Clavibacter michiganense subsp. nebraskense. For each pathogen in each year, hybrids were classified as resistant, moderately resistant, moderately susceptible, or susceptible, according to a categorization procedure based on mean separation tests (Waller-Duncan Bayesian least significant difference) for disease severity. A hierarchical cluster analysis which considered reactions to all diseases in all years grouped 66 hybrids that were included in all three trials into seven major groups, 11 subgroups, and 17 sub-subgroups. Major groups were defined primarily by reactions to northern leaf blight. Stewart''s wilt, and Goss'' wilt and reflected the correlations among hybrid reactions to those diseases. Sub-subgroups were defined primarily by reactions to common rust, which reflected the lack of correlation among hybrid reactions to rust and the other diseases. Yield reductions associated with resistance categories were evaluated by regression analyses. For hybrids classified as resistant or moderately resistant, yield reductions were less than 12%, except for rust and Goss'' wilt in 1986. Seven hybrids that were representative of each resistance category for each disease were selected as standards by which to compare these and other evaluations.