INTERACTION OF HT AND PARTIAL RESISTANCE TO EXSEROHILUM-TURCICUM IN MAIZE

Components of northern leaf blight resistance in maize due to race-spe cific resistance controlled by the Ht gene, partial resistance derived from inbred H99, and a combination of the two kinds of resistance, we re studied subsequent to inoculation with Exserohilum turcicum race O. Lesion types, number of lesions (lesion number), percent leaf area af fected (severity), and area under the disease progress curve (AUDPC) b ased on lesion number and severity were assessed in field studies cond ucted at two locations in Uganda and one location in Ohio in 1993. Les ion types observed were consistent for genotypes across locations. In general, significant differences among genotypes for data based on les ion number and severity were consistent for AUDPC based on lesion numb er and severity, respectively, at all locations. In Ohio, both Ht and partial resistance were effective in limiting disease development. In Uganda, susceptible inbreds (A619, A635, and B73) generally had higher severity than genotypes with partial resistance (H99, Mo17, and Babun go 3). However, there was a difference in response among genotypes dep ending on disease intensity at each location. Ht resistance and modera te partial resistance did not greatly affect lesion number at the high er disease intensity location, compared with the susceptible inbreds, but at the lower disease intensity location genotypes with partial res istance had fewer lesions than susceptible inbreds or the Ht conversio ns of the susceptible inbreds. At both plot locations, genotypes with partial resistance had lower severity than the susceptible inbreds or Ht conversions of the susceptible inbreds. Hybrids derived from crossi ng H99 with genotypes with moderate levels of partial resistance (Mo17 and Babungo 3) did not have significantly lower lesion numbers than h ybrids of susceptible inbreds crossed with H99, but severity was signi ficantly lower on these hybrids at the high disease intensity location . Results indicate that the level of partial resistance in H99 would b e as effective in controlling northern leaf blight as using Ht resista nce, or a combination of Ht resistance and moderate levels of partial resistance as found in Mo17.