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Relationship of Hydroxamic Acid Content in Corn and Resistance to the Corn Leaf Aphid¹

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 x high, intermediate x intermediate, and low x low DIMBOA lines were generally low, intermediate, and high, respectively. Hybrids derived from crossing high x low, high x intermediate, and intermediate x low DIMBOA lines produced infestation values between the parental extremes.

Key Words: Zea mays L. • Maize • Biological resistance • Cyclic hydroxamate • Rhopalosiphum maidis (Fitch)

² Graduate research assistant in plant science, professor of plant science, associate professor of entomology, and professor of plant science, respectively.

Received for publication June 21, 1976.

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