In 1959 the cyclic hydroxamate 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) was first reported in maize and has since been directly implicated in resistance to several pathogens. DIMBOA occurs naturally in the glucosidic form and is converted to the fungitoxic aglucone through mycelial penetration or mechanical injury.

In our study, 16 inbred lines of maize commonly used as parents of many hybrids in the U. S. were utilized to test the relationship of concentration of hydroxamates and resistance to Helminthosporium turcicum in the field. One set of the 16 inbreds was analyzed for hydroxamates non-destructively by a rapid procedure using stem tissue from seedlings 36 to 40 cm high (Long et al., Crop Sci. 14:601, 1974). The same set of inbred lines was similarly analyzed at mid-silking stage by Hamilton's procedure using leaf tissue (J. Agric. Food Chem. 12:14, 1964). A third 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) was obtained between concentrations of hydroxamates in stem tissue from seedlings 36 to 40 cm high and DIMBOA concentrations in leaf tissue at the mid-silking stage. A significant negative correlation (r = -0.57) was obtained between concentrations of hydroxamates in seedling stem tissue and susceptibility to H. turcicum. Similarly, a highly significant negative correlation (r = -0.64) was obtained between concentrations of DIMBOA in leaf tissue from inbreds at the mid-silking stage and susceptibility to H. turcicum. From these data we feel that the rapid procedure provides a reasonable measure of resistance to H. turcicum in the field.

B. J. Long, G. M. Dunn and D. G. Routley

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