Broomrapes (Orobanche and Phelipanche species, Orobanchaceae) are obligate root parasites of dicotyledonous plants. This taxonomic group includes seven weedy parasites of agricultural crops that damage vegetables, sunflower (Helianthus annuus L.), and legumes. Processing-tomato (Solanum lycopersicum L.) fields in Israel have been recently found infested with a new broomrape, first identified as nodding broomrape (Orobanche cernua Loefl.) based on its host. However, its morphology resembled the closely related sunflower broomrape (Orobanche cumana Wallr.), an obligate parasite of sunflower. The new race (CUCE) parasitized sunflower, tomato, and tobacco (Nicotiana tabacum L.) in vitro, in a polyethylene bag system and in pots. Its seeds germinated in response to strigolactones (orobanchol, 5-deoxystrigol, 2′-epiorobanchol, and GR24) and dehydrocostus lactone (DCL), whereas O. cumana seeds responded only to DCL and GR24, and O. cernua only to strigolactones. Based on morphological similarities with O. cumana, shared molecular markers with O. cumana, ability to parasitize sunflower and respond to sunflower-germination stimulants, it was concluded that CUCE is a new race of O. cumana, with a host range expanding to Solanaceae crops. While being an important noxious weed of sunflower, this new O. cumana race is currently spreading and posing a threat to processing tomato in Israel. This finding is an alarming indication that broomrapes can shift host range and that similar new races of O. cumana could potentially appear in other countries.

A soilless unit was devised in which host plant (carrot, tomato) roots can be readily infected with broomrape. The simple, inexpensive and reusable unit consists of two glass plates, synthetic cloth, and foam rubber. The host plants are placed (or grown) between the glass plates, being held against one of the plates by foam rubber. The unit provides proper growing conditions, convenient visibility of host roots and parasite from an early stage of infection, and nondestructive access to the roots and parasite. Exposure of the broomrape seeds to a synthetic germination stimulant (GR 24) prior to their application to the host roots, generally, but not always, caused some increase in the number of parasitic infections and/or the rate of broomrape growth.

The effects of acid scarification, prechilling, summer irrigation, and root extract stimulant from faba bean plants grown during different seasons on crenate broomrape seed dormancy and germination were determined. Immersion of 6- to 12-mo old seed in 0.5 to 5 N H2SO4 solution from 1 to 20 min increased germination by 20%. Similarly, seed pretreatment at temperatures between −4 and 6 C increased germination, regardless of the pretreatment duration. Results suggest that the hard seed coat is an important dormancy mechanism in newly formed crenate broomrape seeds. Summer irrigation did not substantially change the germination capacity of crenate broomrape seeds. The germination stimulant activity of the root extract from faba bean plants grown during different seasons varied considerably, decreasing in this order: autumn > winter > spring > summer. This suggests an ecological adaptation of crenate broomrape to its main host crop.

Small broomrape is a holoparasitic plant that attaches to the roots of red clover as well as several other host plants. Hosts and false hosts produce stimulants that induce small broomrape germination but small broomrape does not attach to a false host. Wheat has been identified as a false host for small broomrape; therefore, studies were conducted to investigate the effect of red clover and wheat root exudates on small broomrape germination. In one study, the effect of exudates from red clover and wheat at multiple growth stages on small broomrape germination was evaluated. Red clover induced small broomrape germination at all growth stages tested but was greatest (78%) in the presence of exudates from red clover at the three-trifoliolate stage. Maximum small broomrape germination was 25% when exposed to exudates produced by one-leaf-stage wheat. In a second study, the relationship between small broomrape germination and host growth condition was evaluated using root exudates from red clover or wheat grown under several temperature conditions for either 4 or 8 wk. For the different temperatures, there were no differences in small broomrape germination when exudates of red clover grown for 4 wk were used. Small broomrape germination was reduced when exposed to exudates from red clover plants grown for 8 wk at 10 C compared with plants grown at 15, 20, and 25 C. Differences in small broomrape seed germination were observed with temperature under which wheat was grown for 4 wk, but not for 8 wk. Although wheat exudates resulted in less small broomrape seed germination than red clover exudates, growing wheat as a false host in a small broomrape-infested field could be an important component of an integrated management plan.