Frequency of the Ht1 gene in populations of sweet corn selected for resistance to Exserohilum turcicum Race 1

The possibility that the Ht1 gene or genes tightly linked to Ht1 convey general resistance to races of Exserohilum turcicum that are virulent against Ht1 (i.e., residual resistance) could be useful in sweet corn where the Ht1 gene is present in many commercial hybrids and breeding populations. The objective of this study was to determine if the frequency of the Ht1 gene changed in populations of sweet corn selected for general resistance to E. turcicum race 1, thus conveying residual resistance. Four populations were developed with theoretical initial frequencies of the Ht1 gene of 0, 0.25, 0.25, and 0.5. The populations were advanced by recurrent mass selection with parental control through four or five cycles of selection following inoculation with an Ht-virulent race of E. turcicum (i.e., race 1). Plants from each cycle of each population were evaluated for severity of northern corn leaf blight (NCLB) and chlorotic lesion reactions following inoculation in field and greenhouse trials with either race 0 or 1 of E. turcicum. Recurrent mass selection for general resistance to E. turcicum race 1 reduced the severity of NCLB in all four populations of sweet corn, although the change in the most susceptible population was minimal. Percent gain per cycle was 14.5, 12.3, 14, and 3.7% for populations I, II, III, and IV, respectively. The Ht1 gene did not convey levels of general resistance to E. turcicum race 1 that were substantial enough to be selected for in this population improvement program. There was no apparent selection advantage for resistance to E. turcicum race 1 in the populations that contained the Ht1 gene. The frequency of the Ht1 gene did not differ among cycles of selection within any of the populations in response to improved levels of general resistance to NCLB. The lack of change in frequency of Ht1 in these populations and the similarity in gain per cycle among populations with and without Ht1 lead us to conclude that the Ht1 gene itself did not have a residual effect on resistance.