Herbicide Placement Site Affects Small Broomrape (Orobanche minor) Control in Red Clover

Abstract

Small broomrape is an annual holoparasitic weed that was recently discovered in red clover production fields in Oregon. Imidazolinone herbicides such as imazamox control small broomrape; however, the mechanism of uptake by the parasite is largely unknown. Studies were conducted to determine the imazamox route of uptake by small broomrape in red clover, and to determine the potential for imazamox to be exuded from red clover and the subsequent effect on small broomrape. Small broomrape control was best at 90% when imazamox was foliar-applied, and worst at 42% or less when imazamox was soil-applied. The presence of activated charcoal to adsorb imazamox at the soil surface did not affect efficacy of broadcast foliar treatment. Small broomrape control was also evaluated when a foliar-treated red clover plant was grown in the same pot as a nontreated, parasitized red clover plant that was bagged during herbicide application. Activated charcoal was spread on the soil surface to adsorb imazamox, thus limiting herbicide uptake routes to the foliage of one of two red clover plants in the pot. Small broomrape attachment decreased on nontreated red clover when the other red clover plant in the pot was treated, suggesting roots exuded the herbicide or an active metabolite.