Resistance to imazethapyr was identified in a population of common sunflower that had been treated with imazethapyr for seven consecutive years. The imazethapyr-resistant biotype of common sunflower was approximately 170 times more resistant to imazethapyr than the susceptible biotype based on the rate required for 25% control. Resistance was due to altered acetolactate synthase (ALS) that is less sensitive to imazethapyr. The imazethapyr concentration required to inhibit in vitro ALS activity by 25% was 210–fold higher in the resistant biotype than in the susceptible biotype. Differences in absorption, translocation, and metabolism of imazethapyr in common sunflower biotypes were not sufficient to explain the resistance to imazethapyr.

The study was conducted to determine the cross-resistance of imazethapyr-resistant common sunflower (Helianthus annuus) to selected imidazolinone, sulfonylurea, and triazolopyrimidine herbicides. Whole-plant herbicide dose–response curves and in vitro enzyme studies showed that imazethapyr-resistant common sunflower was highly resistant to imazamox, slightly resistant to thifensulfuron and chlorimuron, and not resistant to cloransulam. Resistance ratios of herbicide concentrations required to inhibit growth by 25% were 310, 3.3, 2.0, and 1.4 times greater in the resistant biotype than in the susceptible biotype for imazamox, thifensulfuron, chlorimuron, and cloransulam, respectively. Similarly, herbicide concentrations required to inhibit ALS activity in vitro by 25% were 332.0, 18.6, 8.3, and 1.2 times greater in the resistant biotype than in the sensitive biotype for imazamox, chlorimuron, thifensulfuron, and cloransulam, respectively.

Imazethapyr resistance in common sunflower (Helianthus annuus) was confirmed in 1996 in a field near Rossville, KS. In 1997, common sunflower achenes were collected within a 20-km radius of the field with known resistance to determine if resistance was present in nearby fields or if resistance had spread to the native population on the roadside. Collections were made from 14 soybean (Glycine max) fields, one corn (Zea mays) field, and 11 roadsides. Achenes from Konza Prairie Research Natural Area, a prairie that had received no herbicide applications in the past 25 yr, served as the susceptible control. Common sunflower seedlings were treated in a greenhouse with 71 g ai/ha imazethapyr and 11 g ai/ha chlorimuron. In all 15 fields sampled, at least 1% of the common sunflower exhibited an intermediate response to imazethapyr or chlorimuron. In 13 fields, at least 1% of the plants were resistant to imazethapyr, and in all 15 fields, at least 1% of the plants were resistant to chlorimuron. Ten roadsides had common sunflower that showed intermediate response to imazethapyr or chlorimuron. At least 1% of the plants from seven roadsides were resistant to imazethapyr or chlorimuron. Common sunflower collected from fields with repeated applications of imazethapyr showed more resistance to imazethapyr than to chlorimuron.