Abstract:
Small broomrape (Orobanche minor) is a parasitic weed that attaches to the roots of red clover (Trifolium pratense). Small broomrape invasion presents a significant threat to the future of red clover seed production in Oregon. This study was conducted to investigate and develop small broomrape management options for red clover seed production. Experiments were conducted to evaluate herbicide treatments applied after small broomrape emergence in red clover. Imazamox and imazamox plus bentazon treatments were the only herbicide treatments that consistently exhibited a
high level of crop safety, reduced small broomrape density, and did not reduce red clover yield. Herbicide treatments did not prevent production of viable small broomrape seed. Small broomrape seed must be stimulated by host plant exudates for germination and attachment to occur. However, false-host plant species can stimulate parasitic seed germination without attachment. Wheat was a false-host of small broomrape; therefore, experiments were conducted to evaluate the effect of wheat on small broomrape germination. Wheat cultivars induced 20 to 70% of small broomrape seed to germinate. Small broomrape attachment was reduced on red clover plants grown in potting mix
previously planted to wheat compared to where none were grown. Red clover plants also had fewer small broomrape attachments when grown in field soil following wheat compared to plants in soil where no wheat was previously grown. The effect of small broomrape parasitism on the biomass partitioning of its primary host, red clover, has not been documented. An experiment was conducted to determine the relationship between small broomrape and red clover biomass accumulation. Total biomass of parasitized red clover plants and their attached small broomrape was 40% less than the total biomass of non-parasitized red clover plants. Small broomrape parasitism reduced the amount of dry matter allocated to red clover inflorescences by 70%. Red clover establishment with winter wheat in a small broomrape management system was investigated. Red clover that was spring interseeded into wheat produced minimal dry matter and ground cover
by the following summer. Red clover fall interseeded with wheat produced enough ground cover for stand retention in one of two sites. Wheat yield was not affected by row spacing or red clover competition.
