Effects of Qualitative and Quantitative Resistance on the Development and Spread of Northern Leaf Blight of Maize Caused byExserohilum turcicumRaces 1 and 2

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Abstract
The effects of qualitative and quantitative resistance on the development and spread of northern leaf blight (NLB) of maize caused by Exserohilum turcicum races 1 and 2 was studied in 1984 and 1985 in a 2 .times. 2 .times. 2 factorial with the following treatments; the hybrids A619 .times. A632 and B73 .times. Mo17, the genes Ht1 and ht1; and races 1 and 2 of E. turcicum. In 1985, A619Ht2 .times. A632 was also evaluated. NLB severity gradient were studied for each treatment. In both years, host genotype-pathogen reacted combinations were classified into resistant and susceptible groups based on NLB severity and spread. Hybrid-race combinations differed significantly between resistant and suceptible groups but did not differ significantly within groups. Both versions of B73 .times. Mo17 inoculated with either race and A619Ht1 .times. A632Ht1 inoculated with race 1 were resistant. The severity of NLB on resistant hybrids was less than 3% at all sampling time and distances from the foci. All versions of A619 .times. A632 except A619H:1 .times. A63Ht1 inoculated with race 1 were susceptible. In plots of susceptible hybrids, NLB severity was below 20% at all times and distances except for 0.8 m from the foci at 8, 9, and 10 wk after inoculation. High levels of qualitative and quantitative resistance appear to be equally effective in limiting the spread and development of NLB although genes for qualitative resistance may not be effective against all pathogen races and in some hybrid combinations, such as the Ht2 gene in A619 .times. A632. Severe NLB epidemics appear to require large amounts of initial inoculum and/or environmental conditions which result in a abundant secondary inoculum production.