Ephrath, J.E., French Associates Institute for Agriculture and Biotechnology of Drylands, Wyler Department of Dryland Agriculture, Jacob Blaustein Institutes for Desert Research, Sede-Boqer, Israel
Eizenberg, H., Department of Plant Pathology and Weed Research, Agricultural Research Organization, Newe Ya'ar Research Center, Ramat Yishay, Israel

The dynamics of the host-parasite relationship between sunflower and two broomrape species, Orobanche cumana and Phelipanche aegyptiaca, are temperature-related. This relationship can be quantified by thermal time as measured by growing day degrees (d °C; GDD). To obtain a reliable prediction from thermal time values, studies based on a wide range of temperatures are essential. O. cumana was found to be more suited to germination under high temperatures than P. aegyptiaca. The maximal parasite germination percentage, 79% and 49% in O. cumana and P. aegyptiaca, respectively, occurred in the two species in a similar thermal time of about 500 GDD. Although, there was significantly higher germination of O. cumana seeds compared with P. aegyptiaca under the various temperature regimes, there was no effect of temperature on the number of O. cumana attachments, when compared with P. aegyptiaca. The relationship between the progress of O. cumana and P. aegyptiaca attachments and emergence and thermal time followed a sigmoid-shaped curve. O. cumana and P. aegyptiaca biomass was found to be strongly related to thermal time. © 2010 The Authors. Journal Compilation © 2010 European Weed Research Society.