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Consistent differences among maize (Zea mays L.) genotypes for resistance to ,4spergillus flavus Link ex Ft. infection and aflatoxin contamination have been difficult to demonstrate. Populations derived from visibly classified segregating kernels, obtained from a single fungus-infected hybrid ear, were tested for aflatoxin contamination amounts. The objectives were to look for hypothesized differences in contamination between the two selected populations derived from kernels related no less closely than half sibs, and to determine if the differences between these two populations were consistent when samples were inoculated with .4. flavus in the field and laboratory. The population (MAS:gk), expected to be resistant based on the fungus-free appearance of kernels selected from the original ear, had significantly less aflatoxin contamination than its counterpart (MAS:pw,nf) in three of four field experiments and two of three laboratory experiments. The seven replicated experiments were conducted during a 5-yr period, and contamination of MAS:pw,nf exceeded that of MAS:gk in all seven trials. The probability of this occurring as hypothesized, when two populations are in fact equal in resistance, is <0.01. The variation in kernel phenotype on which the initial selections were made has since disappeared, and the populations resulting from sib-poilination do not differ in their resistance to insects nor in several other traits measured thus far. The populations MAS:gk and MAS:pw,nf offer a unique opportunity for genetic improvement for resistance to aflatoxin through selection and for investigating the mechanisms of resistance.

Aflatoxin contamination of maize (Zea mays L.) is a chronic problem in the Southeastern USA, but varies greatly from year to year in its severity. Our primary objective was to assess the effect of planting date on aflatoxin concentrations in grain from knife‐wound inoculated ears during a five‐year study at Tifton, GA. We also evaluated the relationship between aflatoxin concentrations in maize grain and several environmental variables, especially those involving temperature and moisture. Five plantings of Funk's G‐4507 were made in 1982, six in 1983 and 1984, and 10 in 1986 and 1987. Average planting dates varied from about 1 March to 1 August. Eight to 12 wound‐inoculated, 10‐ear samples were assayed per planting date. Aflatoxin concentrations were not significantly related to insect damage or the percentage of ears with visible Aspergillus flavus Link ex Fries, but concentrations were correlated with maximum and minimum daily temperature and net daily evaporation during 20‐ to 40‐d and 40‐ to 60‐d periods following full silk. Combined and average correlations for these relationships were highly significant. Regressions of aflatoxin concentrations on planting date revealed a significant linear decrease in concentration (200–300 ppb per 15‐d delay in planting) from early to late. Simple coefficients of determination of 0.75 or greater also suggest a significant relationship when measured over periods of more than 1 yr. Early plantings in Georgia are believed to be at higher risk because the critical grain filling period, beginning 20 d after silking, falls when the seasonal maximum and minimum temperatures are highest and when net evaporation is at its peak.

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