Abstract:
Mayweed chamomile (Anthemis cotula L.) is an annual weed in the Asteraceae family that is commonly found in fields of the Pacific Northwest. Acetolactate synthase (ALS)-inhibiting herbicides are frequently used to control a broad spectrum of weed species including mayweed chamomile. Seeds of four biotypes of mayweed chamomile (KJ, KL1, KL2, and GW) suspected to be resistant to ALS-inhibiting herbicides were collected from different fields in Washington State, USA. Seeds from a susceptible (S) biotype of mayweed chamomile collected in Oregon were used as the control in all the experiments. Greenhouse studies were conducted to determine if the biotypes were resistant to four chemistry classes of ALS-inhibiting herbicides which were sulfonylureas (SU), imidazolinones (IMI), pyrimidinylthio-benzoate (PTB), and sulfonylamino-carbonyltriazolinone (SCT). A whole-plant dose-response assay confirmed cross-resistance to thifensulfuron+tribenuron (SU) and imazethapyr (IMI) in resistant biotypes. There were high levels of resistance to thifensulfuron+tribenuron and moderate to low levels of resistance to imazethapyr. Propoxycarbazone (SCT) and cloransulam (TP) were applied at the recommended field rate. All resistant biotypes had moderate levels of resistance to propoxycarbazone while biotypes GW and KJ had moderate and low levels of resistance to and cloransulam, respectively. The resistance also was confirmed using an in vitro ALS assay. The herbicide concentrations that inhibited ALS activity by 50% (I50) in the resistant biotypes were between 26- and 289-fold greater than in the S biotype for thifensulfuron+ tribenuron; 2- to 5-fold greater for imazethapyr; 3- to 18-fold greater for propoxycarbazone; and 4- to 18-fold greater for cloransulam. Once resistance was confirmed, the ALS gene was sequenced to determine if mutations occurred in the target-site. At least two ALS isoforms of ALS gene (ALS1 and ALS2) were found in the mayweed chamomile biotypes. No mutations were observed in ALS2. The target-site mutations conferring the resistance to ALS-inhibiting herbicides were likely in ALS1. Sequence analysis of the ALS1 gene identified four point mutations (Pro197 to Leu, Gln, Thr or Ser) at position 197 in the resistant biotypes. Homozygous and heterozygous resistance and the existence of two different mutant ALS alleles were found in resistant biotypes. The ALS-resistant alleles and genotypes in resistant biotypes were diverse and related to the cross-resistance to ALS-inhibiting herbicides.
