Evaluation location: Edirne, Turkey
903 Helianthus accessions were screened for resistance/tolerance to Orobanche at the Thrace Agricultural Research Institute (TARI), Edirne, Turkey. Two replications of single-row plots (5.10 cm in length) were planted April 11, 1993. Rows were spaced 1 meter apart and hills were made within each row at a 30 cm spacing. Although the field was naturally infested, Orobanche inoculum was also applied. The inoculum was prepared with Orobanche seeds collected from sunflower fields in different parts of the Thrace region. The Orobanche seeds were mixed with fine sand in a 1:15 ratio. Approximately 500 mg of inoculum was applied to each hill along with 3 sunflower seeds and the hill was irrigated. After emergence, plants were thinned to one sunflower plant per hill. Just prior to sunflower flowering, Orobanche shoots were beginning to emerge. Orobanche counts were taken prior to sunflower physiological maturity when the maximum number of Orobanche shoots was reached. Data were recorded for stand, number of sunflower plants parasitized by Orobanche, and total number of Orobanche shoots per plot. Frequency (% sunflower plants parasitized) and attacking rate (average number of Orobanche per parasitized sunflower) were calculated. Under Turkish conditions, sunflower varieties with a frequency < 10% and an attack rate < 1 are considered resistant or highly tolerant.