volunteers and haloxyfop

American sloughgrass is a troublesome annual grass weed in winter wheat field rotated with rice in China. The overreliance on acetyl-coenzyme A carboxylase (ACCase) inhibiting herbicides has resulted in resistance evolution in this weed. In this study, the cross-resistance patterns to fenoxaprop-p-ethyl, clodinafop-propargyl, fluazifop-p-butyl, haloxyfop-p-methyl, sethoxydim, clethodim and pinoxaden were established using purified plants individually homozygous for specific mutant ACCase alleles. Results indicated that 1781Leu allele endows high-level resistance to APPs, CHDs and pinoxaden while confers moderate resistance to haloxyfop-p-methyl. The 2027Cys and 2041Asn alleles endow high-level resistance to APPs and pinoxaden and lower level resistance to CHDs. The 2078Gly allele confers high-level resistance to all herbicides tested in this study, however, moderate resistance to sethoxydim. The 2096Ala very likely endows high-level resistance to fluazifop-p-butyl, haloxyfop-p-methyl and moderate resistance to sethoxydim. In addition, one undefined resistance mechanism was involved in population SD-04.

Topics: Acetyl-CoA Carboxylase; Cyclohexanones; Dose-Response Relationship, Drug; Herbicide Resistance; Herbicides; Heterocyclic Compounds, 2-Ring; Oxazoles; Plant Proteins; Poaceae; Propionates; Pyridines

There has been much debate regarding the potential for reduced rates of herbicide application to accelerate evolution of herbicide resistance. We report a series of experiments that demonstrate the potential for reduced rates of the acetyl-co enzyme A carboxylase (ACCase)-inhibiting herbicide diclofop-methyl to rapidly select for resistance in a susceptible biotype of Lolium rigidum. Thirty-six percent of individuals from the original VLR1 population survived application of 37.5 g diclofop-methyl ha(-1) (10% of the recommended field application rate). These individuals were grown to maturity and bulk-crossed to produce the VLR1 low dose-selected line VLR1 (0.1). Subsequent comparisons of the dose-response characteristics of the original and low dose-selected VLR1 lines demonstrated increased tolerance of diclofop-methyl in the selected line. Two further rounds of selection produced VLR1 lines that were resistant to field-applied rates of diclofop-methyl. The LD50 (diclofop-methyl dose required to cause 50% mortality) of the most resistant line was 56-fold greater than that of the original unselected VLR1 population, indicating very large increases in mean population survival after three cycles of selection. In vitro ACCase inhibition by diclofop acid confirmed that resistance was not due to an insensitive herbicide target-site. Cross-resistance studies showed increases in resistance to four herbicides: fluazifop-P-butyl, haloxyfop-R-methyl, clethodim and imazethapyr. The potential genetic basis of the observed response and implications of reduced herbicide application rates for management of herbicide resistance are discussed.

Topics: Crosses, Genetic; Cyclohexanones; Dose-Response Relationship, Drug; Drug Resistance; Evolution, Molecular; Halogenated Diphenyl Ethers; Herbicides; Lethal Dose 50; Lolium; Nicotinic Acids; Phenotype; Phenyl Ethers; Pyridines; Selection, Genetic