REACTIONS OF OPEN-POLLINATED SWEET CORN CULTIVARS TO STEWARTS WILT, COMMON RUST, NORTHERN LEAF-BLIGHT, AND SOUTHERN LEAF-BLIGHT

Over 800 open-pollinated (OP) varieties of sweet corn were grown and n amed in the century prior to the development of hybrids, but only a fe w of the historically important OP cultivars exist today. Alleles that could improve disease resistance of modern sweet corn may be present in the OP cultivars still in existence. The objectives of this researc h were to compare 36 OP sweet corn cultivars to modern commercial hybr ids for reactions to Stewart's wilt, common rust, northern leaf blight (NLB), and southern leaf blight (SLB), and to classify the OP cultiva rs based on phenotypic reactions to these four diseases. Plants were i noculated in 1994, 1995, and 1996 with Erwinia stewartii, Puccinia sou ghi, Exserohilum turcicum, or Bipolaris maydis. Symptoms were rated on a whole-plot basis, and ratings were analyzed by analysis of variance (ANOVA). Means were separated by Bayesian least significant differenc e values. Some of the OP cultivars had phenotypes that were intermedia te to moderately resistant to Stewart's wilt, common rust, NLB, or SLB , but none of the cultivars were more resistant than the best commerci al hybrids. Distributions of ratings for rust, NLB, and SLB were less disperse for the OP cultivars than for commercial hybrids. Hence, the resistance of modern sweet corn germ plasm to Stewart's wilt, rust, an d NLB appears to be greater than that of the OP cultivars. OP cultivar s and four standard hybrids were placed into groups based on a hierarc hical cluster analysis of disease reactions. The seven groups formed f rom the cluster analysis of disease ratings were considerably differen t than those formed from isozyme variation and morphological. characte ristics. The partial resistance of some cultivars, e.g., Golden Sunshi ne, Country Gentleman, Stowell's Evergreen, and Red, may be relatively diverse since these cultivars were placed in different groups based o n isozyme and morphological variation. OP cultivars with moderate leve ls of resistance may be sources of resistance alleles not present in c ommercial hybrids.