Survey of common sunflower (Helianthus annuus) resistance to imazethapyr and chlorimuron in northeast Kansas

Imazethapyr resistance in common sunflower (Helianthus annuus) was confirme d 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 s pread to the native population on the roadside. Collections were made from 14 soybean (Glycine max) fields, one corn (Zea mays) field, and 11 roadside s. Achenes from Konza Prairie Research Natural Area, a prairie that had rec eived no herbicide applications in the past 25 yr, served as the susceptibl e 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, a t lease 1% of the common sunflower exhibited an intermediate response to im azethapyr or chlorimuron. In 13 fields, at least 1% of the plants were resi stant to imazethapyr, and in all 15 fields, at least 1% of the plants were resistant to chlorimuron. Ten roadsides had common sunflower that showed in termediate response to imazethapyr or chlorimuron, At least 1% of the plant s from seven roadsides were resistant to imazethapyr or chlorimuron. Common sunflower collected from fields with repealed applications of imazethapyr showed more resistance to imazethapyr than to chlorimuron.