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Resistance Conferred by the Ht1 Gene in Sweet Corn Infected by Mixtures of Virulent and Avirulent Exserohilum turcicum

The Ht1 gene conveys a chlorotic-lesion resistant reaction in corn infected by avirulent races of Exserohilum turcicum, the causal agent of northern corn leaf blight (NCLB). The widespread deployment of the Ht1 gene in field corn grown in North America since the 1960s resulted in an increased frequency of E. turcicum race 1, which is virulent against the Ht1 gene. The objective of this study was to assess the value of resistance conveyed by the Ht1 gene when initial inoculum consisted of different ratios of virulent and avirulent E. turcicum. Forty-two sweet corn hybrids with the Ht1 gene and 42 sweet corn hybrids without Ht1 were grown in five trials each in 2003 and 2004. In each trial, plants were inoculated with culture suspensions consisting of different percentages of E. turcicum race 0 and race 1, including: 100:0, 90:10, 75:25, 50:50, and 0:100. Severity of NCLB was rated visually from 0 to 100% leaf area infected when plants were about 3 to 4 weeks past the mid-silk growth stage. The Ht1 gene reduced severity of NCLB by as much as one-third when virulent isolates comprised 25% or less of the initial inoculum. Reduction in NCLB severity due to the Ht1 gene was more substantial on hybrids with susceptible backgrounds than on those with general resistance. When virulent isolates comprised 50% of the initial inoculum, NCLB severity was similar for hybrids with and without the Ht1 gene if hybrids had equivalent levels of general resistance (measured as NCLB severity from trials inoculated entirely with race 1). To accurately classify NCLB reactions of maize lines relative to their most probable performance in the United States, inoculum should consist of at least 50% E. turcicum race 1.