Breeding strategies for introgressing diverse scab resistances into adapted wheats
1997
Singh, R.P. | Ginkel, M. van
Scab of wheat, caused by Fusarium spp., is a major constraint in the humid and semi- humid wheat production areas of many countries. The progress in breeding resistant cultivars at CIMMYT and elsewhere has been slow because most known resistances occur in unadapted germplasm, and the resistances are usually quantitative and based on the additive interactions of a few to several minor genes. Three breeding strategies which may enhance the introgression and accumulation of minor genes into adapted wheats are proposed. In the first scheme, the F1 hybrids are developed by crossing two diverse, scab resistant wheats, eg., Ning 7840 from China and Frontana from Brazil. The F1 hybrids are then top-crossed (three-way) with a third parent which is adapted to the target area but may lack adequate scab resistance. A large (between 1000-2000 plants) top-cross population is required to obtain sufficient plants with all four or five minor genes in a heterozygous condition. We propose to either apply only mild selection pressure, or no selection pressure, for scab resistance in early segregating generations (such as F1-top, F2, and F3). However, in the meantime, strong selection pressure must be applied in these early generations for desirable agronomic attributes and resistance to other diseases such as rusts. The selected plants could be maintained as bulk and a large population size should be maintained in each generation. Intense selection pressure for scab resistance should be applied in the later (F4 and F5) generations, when the level of homozygosity is higher, in order to identify a few transgressive segregants having good agronomic type and four to five genes combined for fusarium resistance. A large number of selected F5 plants could be harvested in a pedigree fashion to achieve pure F6 lines, which could be further assessed for resistance to fusarium.
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