Qualitative Resistance to Exserohilum Turcicum Race 2 in Maize (Corn, Genetics)

95 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1984.The fungus Exserohilum turcicum causes northern leaf blight of maize (Zea mays L.) and it is found in most maize growing areas of the world. This disease has been controlled effectively for over 15 years by qualitative and quantitative resistance. In 1979 a new race of the fungus (race 2) was found in the U.S. Corn Belt. This study was designed to evaluate currently available sources of qualitative resistance for their effectiveness against race 2. The gene Ht(,1) confers a resistant lesion-type to race 1 of E. turcicum but a susceptible lesion-type forms when infection is by E. turcicum race 2. However, in both field and greenhouse studies, hybrids with Ht(,1) were more resistant to race 2 than near-isogenic hybrids without Ht(,1). Subsequent studies indicated that the differences between near-isogenic lines were due in part to resistance to race 2 conferred by Ht(,1) as well as to effects of other genes that had been backcrossed with Ht(,1). The study did not separate effects of Ht(,1) from any effects that may be from genes tightly linked to Ht(,1). Three chlorotic-lesion genes for resistance to E. turcicum are known in maize: Ht(,1), Ht(,2) and Ht(,3). Two-way combinations of these genes were compared in two hybrid and in three inbred backgrounds for resistance to race 2 of E. turcicum. Gene Ht(,3) generally provided high levels of resistance. Widespread chlorosis was associated with the Ht(,2) gene. Resistance genes and specific combinations of genes did not condition the same response to infection in all germplasm evaluated. Analysis of three diallel crosses involving four parents each showed specific combining ability effects to be important for lesion length, lesion number and area under the disease progress curve (AUDPC). These same assessments of disease also were made in four hybrids in three environments. Results indicated that lesion length and number as well as visual estimates of disease severity all recorded prior to mid-silking time can accurately predict AUDPC. A fourth gene, HtN, which confers resistance to E. turcicum race 2, functions by increasing incubation period. This gene is relatively difficult to detect under field conditions and therefore difficult to backcross. A technique using leaf inoculations of seedlings was developed that could accurately detect the HtN gene in inbred lines of maize under greenhouse conditions.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD