A seedling bioassay was developed for the rapid diagnosis of resistance to clethodim and fluazifop-P in johnsongrass. The assay was based on differences in the coleoptile length of susceptible (S) and resistant (R) seedlings exposed to clethodim and fluazifop-P in petri dishes for 5 d. Bioassay concentrations of 0.09 mg/L clethodim and 0.18 mg/L fluazifop-P were chosen as discriminant based on rate responses of each biotype to increasing herbicide dose. At 5 d after treatment (DAT), the amounts of clethodim required to reduce coleoptile length by 50% (GR₅₀) for the R and S seedlings were 462.5 and 24.8 mg/L, respectively, resulting in an R:S ratio of 18.7. The fluazifop GR₅₀ values for the R and S seedlings were 618.7 and 17.5 mg/L, respectively, resulting in a R:S ratio of 35.4.

Nomenclature: Clethodim, fluazifop-P, johnsongrass, Sorghum halepense (L.) Pers. #³SORHA.

Additional index words: ACCase inhibitors, herbicide resistance.

Abbreviations: ACCase, acetyl coenzyme-A carboxylase; AOPP, aryloxyphenoxypropionate; CHD, cyclohexanedione.