Yr78 is a recently identified adult plant resistance (APR) gene conferring yellow rust resistance in a CIMMYT bread wheat line PI520108 (= HORK/KALYANSONA). The gene was located on the 6B chromosome and apparently is not very common, at least, in the soft white winter wheat of the USA. Since no seedling resistance gene has been reported in Kalyansona, we decided to test the second parent (HORK) involved in the cross of PI520108. We identified 952 accessions in the CIMMYT germplasm bank carrying HORK as parent in the pedigree. Approximately 12% sample (117 lines) was tested with both, the marker and yellow rust isolates. A validated Yr78 molecular marker, IWA7257-Yr78 (1), permitted the testing of the lines. There were 76% of the sampled lines that were positive for the marker (Yr78+), but with susceptible seedling infection type displayed severity/reaction ranging from 0 to 60M in the field, which were similar to 1MR-60M responses of lines that displayed resistant seedling reactions. The remaining 24% seedling susceptible lines, negative for the Yr78 marker showed responses ranging from 5M to 50MR in the field indicating the presence of other APR genes. CIMMYT lines other than Hork such as Yukon, Tomas Catedral, Warbler, Pheasant, Pfau, Falke, Xequijel 88, Corydon, Dayma 96 and Nahuatl F2000 were positive for the Yr78 marker. Furthermore, 63% of 620 genotyped lines from three recent CIMMYT International Spring Wheat Screening Nurseries were positive the Yr78 marker, indicating that this gene is very common in CIMMYT germplasm. The nurseries were: the 52nd International Bread wheat Screening Nursery (52IBWSN; 60% Yr78+), the 35th Semi-Arid Wheat Screening Nursery (35SAWSN; 64% Yr78+) and the 30th High Rainfall Wheat Screening Nursery (30HRWSN; 70% Yr78+). Greenhouse and field evaluations at the adult stage indicated that Yr78 alone induces a strong chlorosis and necrosis but does not reduce severity sufficiently. Therefore, it should be used in combination with other race specific or slow rusting APR genes such as Yr18, Yr29, Yr30 and Yr46. Even though Yr78 seems to be partially effective in the USA and Mexico with the current population of yellow rust fungus, the effectiveness in other hot spot areas worldwide remains unknown. 018969