Reactions of Open-Pollinated Sweet Corn Cultivars to Stewart’s Wilt, Common Rust, Northern Leaf Blight, and Southern Leaf Blight

The first commercially successful F1 hybrid sweet corn, Golden Cross Bantam, was released by Smith in the early 1930s (17). Today, over 600 sweet corn hybrids are available commercially in the United States and thousands of new hybrids are developed and evaluated each year. These hybrids display a wide range of phenotypic reactions to prevalent diseases (9), indicating a considerable amount of diversity in commercial sweet corn for resistance or susceptibility to Erwinia stewartii (Stewart’s wilt), Puccinia sorghi (common rusts), Exserohilum turcicum (northern leaf blight), Bipolaris maydis (southern leaf blight), and other maize pathogens. Nevertheless, the elite sweet corn germ plasm used presently in commercial hybrids probably is more similar genetically than the germ plasm composed of hundreds of open-pollinated (OP) cultivars grown prior to the development of hybrids. Some authors in the popular press, such as Pollan (13), contend that the uniformity resulting from a field of F1 hybrid sweet corn plants “violates one of nature’s cardinal principles: genetic diversity” and suggest that “a field of genetically identical plants is much more vulnerable to disease.” While an F1 hybrid is less variable than an OP cultivar, the likelihood of a catastrophic epidemic in today’s sweet corn crop relative to a crop based on OP cultivars depends on factors other than variation within the cultivar. Two of these factors include levels of disease resistance and the genetic basis of resistance. Elite commercial hybrids may be more genetically uniform than their OP ancestors for horticultural and agronomic traits of importance, but the genetic basis of resistance among modern hybrids may be more or less diverse than that of OP cultivars. Over 800 OP varieties of sweet corn were grown and named in the century prior to the development of hybrids (19). Many of these OP cultivars originated from the northeastern United States (19). Commercial development of OP cultivars ended in the United States in the 1930s with the monumental success of Golden Cross Bantam and other sweet corn hybrids, due in part to the moderate Stewart’s wilt resistance of these hybrids. Today, only a few of the historically important OP cultivars exist. These are maintained by organizations such as the North Central Regional Plant Introduction Station (NCRPIS) at Ames, Iowa, and the Seed Savers Exchange (1). Little is published about the reactions of these cultivars to prevalent diseases other than Stewart’s wilt. Alleles that could improve disease resistance of modern sweet corn may be present in some of the OP cultivars still in existence. Some of these resistance alleles may not occur in modern elite germ plasm since historically important inbred lines of sweet corn were developed from only a few OP cultivars, of which Golden Bantam, Stowell’s Evergreen, and Country Gentleman were most prominent (5,6,20). In order to use the disease resistance that may occur in the OP germ plasm, the disease reactions of these cultivars must be identified and characterized. The objectives of this research were to compare OP sweet corn cultivars to modern commercial hybrids for their reactions to Stewart’s wilt, common rust, northern leaf blight (NLB), and southern leaf blight (SLB), and to classify the OP cultivars based on phenotypic reactions to these four diseases.