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Reaction of Diallel Crosses of Maize In T and N Cytoplasms to Bipolaris maydis Race T¹

Diallel crosses among eight corn (Zea mays L.) inbreds that differed for their response to Bipolaris maydis (Nisikado and Miyake) Shoemaker race T (Helminthosporium maydis Nisikado & Miyake race T), were made both on Texas male sterile (T) and on normal (N) cytoplasms. These crosses were evaluated for their reaction to inoculation with B. maydis race T both as seedlings in the greenhouse and as plants in the field.

Single crosses produced in N cytoplasm were heavily infected by spores of the fungus in greenhouse tests but extensive lesions did not develop, and the fungus caused relatively little damage. Differences for disease ratings among the hybrids in the N cytoplasm were not significant either in the greenhouse or in the field. Crosses in the T cytoplasm responded differentially to this fungus. Except for crosses with Mp305 as one of the parents, the average disease severity for all crosses with a given parental line was as expected, based on the original evaluation of the parents as lines. However, disease ratings on the crosses in the greenhouse did not agree well with those obtained in the field.

Crosses produced in T cytoplasm yielded 69% of their normal counterparts in 1972 and 22% in 1973. Differences were noted among hybrids produced in the T cytoplasm. The most resistant hybrid yielded 63% of its normal counterpart in 1973 while the most susceptible hybrid yielded only 1% of its normal counterpart. Thus, we found that necular gene resistance could overcome part, but not all, of the susceptibility associated with T cytoplasm to this disease.

Additive genetic variances accounted for most of the differences among hybrids produced in T cytoplasm. However, nonadditive and reciprocal variances were significant for some characteristics.

Key Words: Zea mays L. • Genetics • Breeding for disease resistance • Yield losses • Helminthosporium maydis

² Research agronomist, ARS-USDA and professor of agronomy Mississippi State Univ., and research plant pathologist, ARS-USDA and professor of plant pathology and weed science, Mississippi State Univ., respectively. Dr. Futrell is deceased.

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