Inheritance of resistance to Orobanche cernua Loefl. in six sunflower lines

Sunflower (Helianthus annuus L.) is severely affected by broomrape (Orobanc he cernua Loen.) in the main crop areas of the Old World. The appearance of new virulent broomrape populations has prompted the search for new sources of resistance. The objectives of this study were to elucidate the inherita nce of sources of resistance From different origins and to determine alleli c relationships between the resistance genes. Six resistant sunflower lines (one of them with resistance gene Or(5)), two susceptible lines, the F-1 c rosses between resistant and susceptible lines and resistant and resistant lines, and the F-2 and BC1 generations were evaluated for broomrape resista nce with the widespread highly virulent population SE 194. Genetic ratios f rom segregating generations indicated that resistance to O. cernua in these lines was conferred by a single dominant gene. None of the crosses between resistant lines produced susceptible F-2 or BC1 plants. However, the react ion of the resistant lines to broomrape populations from different areas an d years showed differences to new highly virulent populations. Only two lin es were resistant to all populations, indicating that resistance in these l ines was conferred by additional dominant alleles at the Or locus or by a c luster of very tightly linked non-allelic genes, The resistance found in th e two cultivated lines against the new populations, which overcome the Or(5 ) resistance gene, is an important finding and will aid the development of new resistant cultivars since the current resistant hybrids are based on th is gene. Results from this study can also be used to establish. differentia l lines against the new broomrape populations.