Laboratory experiments were carried out under aseptic conditions to study the effect of nitrogen applied as ammonium nitrate at 0, 20,40,60,80 and 100 ppm (wt per vol) in either distilled H2O or modified Hoagland's solution on germination and radicle length of branched broomrape in association with growing seedlings of different crops. As nitrogen rate increased, seed germination and radicle length of branched broomrape decreased linearly in flax, lentil, pepper, tomato, and wheat in modified Hoagland's solution. Similar responses to nitrogen application were found with distilled water. The parasite's seed germination was considerably lower in the media with wheat than with other crops, and lower still in the absence of crop seedings.

Ten herbicides were evaluated for broomrape (Orobanche ramosa L. # ORARA) control and their effect on growth, yield, and chemical composition of four tobacco (Nicotiana tabaccum L.) cultivars. Of the preplant-incorporated (PPI) herbicides, only pebulate (S-propyl butylethylcarbamothioate) at 7.2 kg ai/ha and metham (methylcarbamodithioic acid) at 66 kg ai/ha provided, although inconsistently, 30 to 50% broomrape control in oriental but not in burley tobacco. Single postemergence (POE) applications of glyphosate [N-(phosphonomethyl)glycine] at 0.2 kg ai/ha or MH (1,2-dihydro-3,6-pyridazinedione) at 0.7 kg ai/ha at 40 days after transplanting resulted in 60 to 80% broomrape control. Glyphosate or MH applications at 40 days after transplanting and again at 60 days resulted in 100% and 80 to 90% control, respectively. Very good (80 to 90%) control also occurred with pebulate (PPI) at 7.2 kg ai/ha followed by glyphosate (POE) at 0.2 kg ai/ha at 40 days after transplanting. Tobacco yield increased significantly, compared to that of untreated plots, where glyphosate or MH was used, but nicotine, total nitrogen, and reducing sugar contents were not significantly affected.

Broomrapes are parasitic angiosperms that live on the roots of many important crop plants. We have conducted a series of in vitro experiments to investigate the direct effects of nitrogen fertilizers on broomrape seed germination and early development. These experiments confirm that nitrogen in the ammonium form is more inhibitory than nitrate, but we conclude that it is the elongation of the seedling radicle that is primarily inhibited by ammonium, rather than the seed germination itself. Reductions in radicle length were observed when ammonium solutions were applied during either preconditioning or germination periods. For germinating seeds, exposures to ammonium sulfate of 4 to 8 h (depending on the species) reduced radicle elongation by half, indicating a relatively rapid inhibition. Broomrape species varied in sensitivity to ammonium, with Orobanche aegyptiaca being the most tolerant, followed by O. crenata, O. ramosa, O. cernua, and finally O. minor as compared in the presence of 5 mM ammonium. However, all broomrape species were much more sensitive to ammonium toxicity than were host crop species under similar treatment conditions.

Five broadleaf crops, alfalfa, peanut, soybean, tobacco, and tomato, were tested for their susceptibility to three broomrape species, branched, Egyptian and crenate, in the greenhouse. All five crop species were parasitized by one or more broomrape species. Peanut plants were most susceptible and soybean plants were least susceptible to parasitism by broomrape. Egyptian and branched broomrapes had a wider host range affecting plants in both the Solanaceae and Fabaceae. Crenate broomrape mainly infected plants belonging to Fabaceae, except for the few attachments to tomato and tobacco. Apparently, this is the first report on parasitism of peanut plants by branched broomrape and that of peanut and alfalfa plants by crenate broomrape. Parasitism of soybean plants by any of the broomrape species also was observed for the first time in these experiments. These results indicate that broomrapes, although not a major problem on any of the crops in the United States, could parasitize several major broadleaf crops should the parasite become established.

Parasitic plants, including the root holoparasites Orobanche spp., cause devastating damage to crops worldwide. Arabidopsis thaliana is widely used as an amenable model plant system to study host–pathogen interactions. Understanding the molecular basis involved in host–parasite interactions will provide practical tools for the detection of genes responsible for incompatibility and resistance responses. In preliminary petri dish experiments, A. thaliana induced seed germination of O. aegyptiaca, O. minor, and O. ramosa at the rate of 87, 72, and 67% of maximum seed germination, respectively. Arabidopsis thaliana induction of O. crenata and O. cumana seeds was negligible (less than 2% of maximum germination). In additional polyethylene bag studies, A. thaliana was parasitized by O. aegyptiaca, O. ramosa, and O. minor resulting in 12, 5, and 2 parasites per host plant, respectively. The results facilitate the use of A. thaliana in host-parasitic plant interaction research.