Inheritance of Mature Plant Resistance to Helminthosporium maydis Race 0 in Maize

Inheritance of resistance of inbred NC250 of maize (Zea mays L.) to southern corn leaf blight incited by Helminthosporium maydis Nisik. & Miy. race 0 was investigated using seedlings and mature plants. Reactions of seedlings artificially inoculated in greenhouse tests indicated resistance was recessive and ratings of three parents were B73 susceptible, B73rhm resistant, and NC250 segregating. Mature plant ratings by contrast were B73 and B73rhm susceptible and NC250 resistant. Most mature plant evaluations were made under severe natural epiphytotics in south Florida where 25 to 50% of the foliage of susceptible cultivars was usually destroyed by flowering time. Parents, F₁, F₂, F₃, and backcross generations were rated on a scale of 1 (resistant) to 5 (susceptible) in increments of 1.0 or 0.5. Generation mean and diallel analyses indicated mainly additive effects for reaction to H. maydis race 0. Resistance conditioned by rhm-gene was generally satisfactory to about flowering and then declined in effectiveness; however, rhm-gene influenced mature plant resistance as demonstrated by comparisons between 200 F₃ families in each of two populations: F₃'s from B73rhm ✕ NC250 and B73 ✕ NC250 averaged 2.2 and 3.3 for leaf blight ratings and 48.5 and 10.0 for the percentage of families in the three most resistant classes, respectively. These comparisons demonstrated enhancement of mature plant resistance by a gene for resistance in the seedling stage. Segregation ratios for F₂ plants and F₃ families indicated two recessive complementary loci for resistance while quantitative analyses estimates suggested this interpretation was an over simplification. The mature plant resistance of NC250, which persisted through the reproductive stage to near physiological maturity, was largely independent of reaction in the seedling stage, was not complex, and maybe transferred by backcrossing.