Cross-Resistance to Imazethapyr in a Fluazifop-P-butyl-Resistant Population of Digitaria sanguinalis

A population of Digitaria sanguinalis selected by and resistant to the acetyl-coenzyme A carboxylase-inhibiting herbicide fluazifop-P-butyl is cross-resistant to the acetolactate synthase (ALS) inhibitor imazethapyr. The resistant population was six-fold resistant to imazethapyr despite never having been exposed to any ALS-inhibiting herbicide. ALS activity extracted from the resistant population was susceptible to imidazolinone herbicides. Malathion, a known cytochrome P450 monooxygenase inhibitor, synergized imazethapyr activity on the resistant population, suggesting that resistance is due to increased herbicide metabolism. In contrast, malathion antagonized fluazifop-P-butyl activity. Therefore, it is likely that the enzyme responsible for fluazifop acid detoxification in D. sanguinalis is different from the enzyme that detoxifies imazethapyr.