The effect of the sulfonylurea herbicides, bensulfuron, chlorsulfuron, nicosulfuron, primisulfuron, rimsulfuron, thifensulfuron, and triasulfuron, on seed germination and radicle elongation of the parasitic plant, Egyptian broomrape, was tested in vitro without a host plant. These herbicides applied to the seeds at the preconditioning and germination stages significantly reduced radicle elongation of the parasite. Chlorsulfuron, rimsulfuron, and triasulfuron were further tested in polyethylene bags, where the parasite was able to parasitize tomato roots. Chlorsulfuron and triasulfuron applied at 2.5 and 25 μM ai and rimsulfuron at 5.0 μM ai, at the preconditioning and germination stages, almost completely inhibited parasite development, but at lower concentrations the parasite partially recovered 26 days after application. Chlorsulfuron inhibited parasite development for 34 d. Chlorsulfuron, triasulfuron, and rimsulfuron damaged parasite organs and caused rapid death of Egyptian broomrape tubercles when applied after the parasite had completed attachment to host roots. However, chlorsulfuron and triasulfuron at 25 μM ai were toxic to the tomato plants.

Experiments were conducted to investigate the possibility that rhizobacterial nodulation in legumes influences parasitic attack by broomrape. Small broomrape (Orobanche minor), which is commonly associated with swellings of the host root, was studied on red clover (Trifolium pratense) in the presence or absence of Rhizobium leguminosarum bv. trifolii inoculum and was compared to Egyptian broomrape (Orobanche aegyptiaca), which is not associated with host root swellings. Small broomrape showed greater percent seed germination and formed a greater number of attachments on red clover inoculated with rhizobacteria in comparison with noninoculated plants. In contrast, inoculum did not appear to enhance Egyptian broomrape seed germination or the number of its attachments on the host roots compared to noninoculated controls. In aseptic culture, the addition of Rhizobium increased the success of small broomrape in parasitizing clover but was not essential for parasitization. No rhizobacteria were detected in cultures derived from tubercles under septic or aseptic conditions. Morphological observations of small broomrape attachments on red clover suggest that parasitic attachments were not situated over the bacterial nodules but may involve parasite-induced protrusion of host plant root cortex. These results indicate that although rhizobacterial nodulation is not required for parasitization, the presence of nodules facilitates small broomrape germination and attachment to red clover.

Field experiments were conducted on a silty clay loam in Corvallis, OR during the summers of 1995 and 1996 to study the effects of soil solarization, spring-planted green manure crops, fumigation with metham, and combinations of these treatments on annual bluegrass seed survival. Annual bluegrass seeds were incorporated into the soil as a bioassay species and soil samples extracted to a depth of 15 cm to determine effects on seed survival. Soil solarization was applied over a 53- or 59-d period using a 0.6-mil clear polyethylene film. Soil samples were collected from four depths after the solarization period in both solarized and nonsolarized plots and surviving seeds germinated in a greenhouse. Maximum soil temperatures recorded at 5-, 10-, and 20-cm depths were 52, 47, and 33 C in solarized soil, respectively. Solarization reduced annual bluegrass seed survival from 89 to 100% in the upper 5 cm of soil, but did not reduce survival below 5 cm. Solarization may have enhanced seed survival below 5 cm. Cover crops of barley, rapeseed, and sudangrass generally increased survival of annual bluegrass seeds buried 2.5 to 15 cm deep in the soil. Green manure cover crops plus solarization did not improve the efficacy of solarization alone and in some cases diminished the effectiveness of solarization. Solarization significantly improved the efficacy of one-quarter rates of metham (230 L/ha) in the top 5 cm of soil, reducing overall annual bluegrass seed survival in the soil by 40% compared with metham alone (230 L/ha) but only 30% compared with solarization alone. The conventional rate of metham (930 L/ha) was the most effective and consistent treatment across all depths.