The cyclic hydroxamate, 2,4‐dihydroxy.7‐methoxy‐l,4‐benzoxazin‐ 3‐one (DIMBOA), has been implicated in the resistance of corn (Zea mays L.) to fungi and insects. In our study, bioassay and field experiments tested the relationship between hydroxamate concentration and resistance to corn leaf aphid (Rhopalosiphum maidis (Fitch)).In the bioassay, various concentrattons of DIMBOA were added to an artificial diet fed to corn leaf aphids. DIMBOA concentrations of 0.1, 0.25, and 0.5 mg/g diet caused 5.1, 12.8, and 20.8% mortality, respectively. In field trials, 12 inbred lines were evaluated for corn leaf aphid resistance under natural infestation. Aphid infestation was evaluated at mid‐silking stage with a visual rating scale and index system. Cyclic hydroxamate concentration in a separate set of the same lines was estimated by a rapid procedure based upon the colorimetric reaction of hydroxamates with ferric chloride. Aphid infestation values ranged from 119 to 366. Hydroxamate concentration ranged from 0.08 to 1.48 mg/g fresh weight. A highly significant correlation (r = 0.72) was obtained between aphid infestation and hydroxamate concentration.Eighteen hybrids derived from crosses among 12 inbred lines were also evaluated for corn leaf aphid resistance in the field. Aphid infestation values from high ✕ high, intermediate ✕ intermediate, and low ✕ low DIMBOA lines were generally low, intermediate, and high, respectively. Hybrids derived from crossing high ✕ low, high ✕ intermediate, and intermediate ✕ low DIMBOA lines produced infestation values between the parental extremes.

A cyclic hydroxamate (2,4‐dihydroxy‐7‐methoxy‐l,4‐benzoxazin‐3‐one; DIMBOA) recently has been implicated in the resistance of maize (Zea mays L.) to fungi and insects. In our study, a set of 13 inbreds was used to determine the relationship of concentrations of DIMBOA and resistance to northern corn leaf blight (Helminthosporium turcicum Pass).DIMBOA concentration was estimated by the colorimettic procedure of Hamilton in plants 60 to 75 cm in height. Mean concentrations of DIMBOA ranged from 0.06 mg/g to 0.66 mg/g fresh weight. Percent leaf infection on plants at the same height ranged from 7.41 to 19.04%. A significant correlation (r =−0.61) was obtained between these two traits.DIMBOA concentrations also were determined in root, stem, and whorl tissues from five selected inbreds at various stages of growth. At 13‐cm plant height, samples from the inbreds contained uniformly high amounts of DIMBOA. The earliest acceptable separation of high and low DIMBOA lines was obtained using stem or whorl tissue from seedlings 35 to 40 cm in height. Generally those lines highest in DIMBOA at 35 to 40 cm plant height were correspondingly high in DIMBOA at the silking stage. Similarly, lines low in DIMBOA at 35 to 40‐cm plant height were low in DIMBOA at the silking stage.

In 1959, the cyclic hydroxamate 2,4‐dihydroxy‐7‐methoxy‐l,4‐benzoxazin‐3‐one (DIMOBA) was first reported maize (Zea mays L.) and has since been implicated in resistance to several pathogens. In our study, 14 inbred lines of maize commonly used as parents of many hybrids in the United States, plus the genotypes BxBx and bxbx were utilized to test the relationship of concentrations of hydroxamates and resistance to Helminthosporium turcicum Pass. in the field. One set of the 16 inbreds was analyzed for hydroxamates nondestructively by a rapid procedure using steam tissue from seedlings 36 to 40 cm high. The same set of inbred lines was similarly analyzed at mid‐silking stage by Hamilton's procedure using leaf tissue. A second set of the 16 inbred lines was inoculated in the field with spore suspensions of H. turcicum and evaluated for resistance at mid‐silking stage using a visual rating scale.A highly significant correlation (r = 0.76, dr. = 43) was obtained between concentrations of hydroxamates in stem tissue from the seedlings and hydroxamate concentrations in leaf tissue at mid‐silking stage. A significant correlation (r = 0.57, dr. = 46) was obtained between concent.rations of hydroxamates in seedling stem tissue and resistance to H. turcicum. Similarly, a highly significant correlation (r = 0.64. df. = 43) was obtained between hydroxamate concentrations in leaf tissue of inbreds at mid‐silking stage and resistance to H. turcicum.

A rapid procedure was developed for estimating the concentration of a cyclic hydroxamic acid 2,4‐dihydroxy‐ 7‐methoxy‐l,4‐benzoxazin‐3‐one (DIMBOA) for potential use in a maize (Zea mays L.) breeding program. Samples (0.2 to 0.5 g stem tissue) were collected from seedlings at 36 cm to 41 cm extended leaf height. The rapid procedure, essentially a simplification of the colorimetric procedure of Hamilton, was significantly correlated with the latter (r = +0.82) based on the analysis of 13 inbred lines of maize.

Hydroxamic acids have been implicated in the resistance of corn (Zea mays L.) to both fungi and insects. In this study, five selected crosses were used among the four inbreds BxBx, bxbx, B49 and B37 to study inheritance of hydroxamates. Hydroxamate concentration in the parental, F1, F2 and backcross generations for each cross was estimated by a rapid procedure based upon the colorimetric reaction of hydroxamates with FeCl3. Components of variance and estimates of heritability were obtained by the procedures of Warner (1952). F2 and backcross data indicated that concentration of hydroxamates is controlled monogenically in the cross bxbx × BxBx and polygenically in the crosses bxbx × B49 and bxbx × B37. Estimates of gene number using the Castle-Wright formula indicated that hydroxamate concentration is conditioned in B49 and B37 by five and two loci, respectively. The addition of BxBx to either B49 or B37 increased the frequency of genotypes in F2 with a high concentration of hydroxamates. Additive genetic variance was the most important component of the phenotypic variance and resulted in estimates of heritability from 0.64 to 0.79. However, the dominance component of variance was considerably higher for crosses involving BxBx than for the crosses bxbx × B49 and bxbx × B37.