Factors affecting differential sensitivity of sweet corn to HPPD-inhibiting herbicides

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 doseresponse 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.