Sunflower (Helianthus annuus) Response to Diclofop

Abstract

Experiments were conducted in the field, greenhouse, and controlled environmental chambers to determine the injury to sunflower (Helianthus annuusL.) from diclofop (2-[4-(2,4-dichlorophenoxy) phenoxy] propanoic acid} as influenced by diclofop rate, sunflower line and growth stage, and temperature/relative humidity (RH). Eighteen sunflower lines varied widely in response to diclofop applied postemergence at 2.2 kg/ha with visual injury ratings ranging from 5 to 46%. Neither diclofop-tolerant (Hybrids 894 and 903) nor diclofop-susceptible (Hybrid 923 and Inbred HA290) sunflower lines were injured by diclofop at 10 C. Diclofop injured all four sunflower lines at 30 C. Sunflower injury following diclofop application at 30 C was greater at 90% RH than at 40% RH; and the diclofop-susceptible lines were injured more than the diclofop-tolerant lines. Diclofop injury to sunflower at the two- to four-leaf stage was greater than at any other growth stage in the field and greenhouse. Seed yields of sunflower (Hybrid 894) treated with diclofop in the two- and four-leaf stage was reduced 15 and 12% compared to that of untreated weed-free sunflower, respectively, averaged over diclofop rates.