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.

Orobanche aegyptiaca is a chlorophyll-lacking holoparasite that subsists on the roots of plants, inflicting severe damage to legumes and other crops in the Mediterranean region. Common vetch and purple vetch are important legume forage crops in Israel and the Middle East. Experiments in pots containing soil inoculated with O. aegyptiaca seeds have shown that common vetch genotypes ‘Yovel’ and ‘473-A’ are susceptible to the parasite, whereas purple vetch genotypes ‘Popany’ and ‘Sadot’ are resistant. When grown in association with O. aegyptiaca in polyethene bags, purple vetch genotypes stimulated a significantly higher rate of parasite seed germination than that stimulated by common vetch genotypes. However, the number of successful Orobanche attachments developed on the host root was lower on resistant genotypes compared to susceptible ones. This differential response between purple and common vetch genotypes was evident at diverse temperature regimes (17/12, 22/17, and 27/22 C day/night). Microscopic observations revealed that resistant genotypes rapidly developed necrotic lesions surrounding the contact point of the parasite radicle (germ tube) with the host root, hence preventing further development of the parasite. In the sensitive genotypes, attachments developed without necrotic lesions. These data may indicate involvement of host defense mechanisms induced by the parasite attachment.

Broomrapes (Orobanche spp.) are chlorophyll-lacking root parasites of many dicotyledonous species that cause severe damage to vegetables and field crops worldwide. Egyptian broomrape is common throughout Israel, where it parasitizes a wide range of crops. The main damage caused by this parasite is to processing tomatoes. The objectives of this study were to determine (1) the relationship between the parasitism process on tomato roots and Egyptian broomrape seed depth, and (2) the effect of herbicide application on Egyptian broomrape control at depths of 0 to 30 cm. The study was conducted in controlled conditions in 25-L pots containing soil that was artificially infested with Egyptian broomrape seeds. Seeds (1-cm layer) were infested at soil surface and at depths of 6, 12, 18, 24, and 30 cm. Sulfosulfuron was applied PRE to Egyptian broomrape shoots, 14 and 42 d after tomato planting. Egyptian broomrape emergence was highly correlated to the depth of the seed infestation: the deeper the seeds were buried, the more pronounced the delay in Egyptian broomrape emergence. A four-parameter sigmoid equation was found that describes the relationship between Egyptian broomrape parasitism and seed depth. No Egyptian broomrape shoots were observed aboveground when sulfosulfuron was applied on tomato foliage. However, live and dead attachments were observed at different levels in all seed-depth treatments. No significant difference in the efficacies of sulfosulfuron at rates of 37.5 and 75 ai ha−1 was observed at depths of 6, 12, and 18 cm. The best control was obtained at a depth of 6 cm at both sulfosulfuron rates. At depths of 24 and 30 cm, no broomrape control was achieved at 37.5 ai g ha−1 sulfosulfuron, and only low control efficacy was obtained at 75 ai g ha−1.

Sunflower broomrape and Egyptian broomrape are two holoparasites of sunflower that cause severe yield loss. The aim of the current study was to clarify the relationship between temperature and sunflower broomrape and Egyptian broomrape parasitism of the resistant sunflower variety ‘Ambar’ (R) and the susceptible variety ‘Adi’ (S). Experiments were conducted in pots under controlled conditions in a multiclimate greenhouse at five temperature (day–night) regimes: 17:9 C, 20:12 C, 23:15 C, 26:18 C, and 29:21 C. The resistant confectionery sunflower variety (R) was susceptible to sunflower broomrape only at low temperature regimes of 17:9 C, whereas the S variety was highly susceptible from 29 to 9 C. Temperature correlated negatively with the number of sunflower broomrape and Egyptian broomrape shoots that emerged above the soil in the R variety, and positively in the S variety. The number of degenerated tubercles found on the roots of the resistant variety correlated positively with temperature in both sunflower broomrape and Egyptian broomrape. No such correlation could be found in the susceptible sunflower variety grown in the presence of these two parasites. The resistance response of the R variety to sunflower broomrape and Egyptian broomrape was similar regardless of temperature regime. Parasite seeds were successfully germinated and attached to the sunflower roots but the parasite at the infection site degenerated and died. On the other hand, the susceptible variety (S) was equally susceptible to Egyptian broomrape and sunflower broomrape in all the temperature regimes tested, and susceptibility increased as temperature increased.