Characterization of Imazamox-Resistant Shattercane (Sorghum bicolor L.) Populations from Kansas

Shattercane is a problematic summer annual grass weed species in grain sorghum producing regions, including Kansas. Three shattercane populations (DC8, GH4, and PL8) collected from sorghum fields from northwestern Kansas survived the field-use rate (52 g ha-1) of postemergence applied imazamox. The main objectives of this research were to (1) confirm and characterize the level of resistance to imazamox in those putative imazamox-resistant (IMI-R) shattercane populations, (2) investigate the underlying mechanism of resistance, and (3) determine the effectiveness of postemergence herbicides for controlling IMI-R populations. A previously known imazamox susceptible (SUS) shattercane population from Rooks County, KS was used. All three putative populations exhibited a 4.1- to 6.0-fold resistance to imazamox compared to the SUS population. The ALS gene sequences from all IMI-R populations did not reveal any known target-site resistance mutations. A pretreatment of the cytochrome P450-inhibiting insecticide malathion followed by imazamox at various doses reversed the resistance phenotype of PL8 population. In a separate greenhouse study, the postemergence treatments of nicosulfuron, quizalofop, clethodim, and glyphosate provided ≥96% injury to all IMI-R populations. The lack of known ALS target-site mutations and the reversal of resistance phenotype by malathion suggest the possibility of metabolism-based resistance to imazamox in PL8 shattercane population.