Tembotrione inhibits 4-hydroxyphenyl-pyruvate-dioxygenase (HPPD) and was recently registered for use in all types of corn. Some sweet corn hybrids are killed by tembotrione, yet a mechanistic understanding of sensitivity has not been reported. Sensitivity of mesotrione, another HPPD-inhibitor, is conditioned by a single allele. Two hypotheses were tested: (1) response to tembotrione and mesotrione are conditioned by alleles at the same or closely linked loci and (2) the extent of early-season injury from tembotrione and mesotrione is similar on hybrids. The first hypothesis was tested by comparing responses to tembotrione and mesotrione in 136 F3:5 families derived from a cross of mesotrione-sensitive and mesotrione-tolerant sweet corn inbreds. F3 families cosegregated for responses to tembotrione and mesotrione: 94% of the families had the same response to both herbicides. Thus, the same gene or very closely linked genes condition response to both herbicides. On the basis of chi-square goodness of fit tests, responses of families to tembotrione fit a 3 : 2 : 3 sensitive : segregating : tolerant ratio (P = 0.24), which would be expected if sensitivity to tembotrione was conditioned by a single recessive allele. The second hypothesis was tested in six field experiments by quantifying the extent of early-season injury to 249 sweet corn hybrids 1 wk after treatment (WAT) of tembotrione (184 g ha−1) or mesotrione (210 g ha−1). One hundred ninety-three hybrids were tolerant to both herbicides. Seven sensitive hybrids that were severely injured by both herbicides 1 WAT differed in their response 3 to 4 WAT; sensitive hybrids treated with mesotrione had apparently resumed normal growth, whereas those treated with tembotrione died. Conversely, hybrids with intermediate levels of injury (> 10 to 50%) 1 WAT with mesotrione had no visual symptoms of injury from applications of tembotrione. Despite the common genetic basis for response to mesotrione and tembotrione, hybrids with sensitive or intermediate responses to mesotrione did not have identical responses to tembotrione.

Mutation of a cytochrome P450 (CYP) allele on the short arm of chromosome 5 affects sensitivity in sweet corn to mesotrione and to tembotrione plus isoxadifen applied POST. Hybrids that are homozygous for the functional allele (i.e., CYPCYP) are rarely injured at registered use rates, hybrids that are homozygous for mutant alleles (i.e., cypcyp) are frequently injured, and hybrids that are heterozygous for a functional and mutant allele (i.e., CYPcyp) have more variable responses over trials. The objectives of this work were (1) to conduct side-by-side comparisons of sweet corn hybrid responses to mesotrione, tembotrione plus isoxadifen, and topramezone under field conditions; and (2) to compare dose–response relationships among CYPCYP, CYPcyp, and cypcyp hybrids. Among 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors used POST in sweet corn, topramezone was safe on the 746 hybrids tested. When environmental conditions favored crop growth, mesotrione injured the largest number of hybrids, and these hybrids were almost exclusively cypcyp or CYPcyp. The safener isoxadifen added to the tembotrione product greatly reduced occurrence of injury to the CYPcyp genotypic class but not to the cypcyp hybrids. Despite a common genetic basis for herbicide metabolism, genotypic classes of sweet corn hybrids did not have identical field responses to mesotrione, tembotrione plus isoxadifen, and topramezone.