How can field selection for Striga resistance and tolerance in sorghum be improved?

Abstract

Breeding for high yielding Sorghum bicolor varieties with effective resistance and tolerance against the hemi-parasitic weed Striga hermonthica requires suitable selection measures for both characteristics. The objective of this research was to develop a set of practical selection measures that contains independent, reliable and discriminative criteria for resistance and tolerance. Ten sorghum genotypes (CK60-B, CMDT39, E36-1, Framida, IS9830, N13, Seredo, Serena, SRN39 and Tiémarifing) were grown in the field with and without Striga infestation in a split-plot design in 3 successive years (2001-2003) using different Striga infestation levels (low, high and intermediate). Resistance against Striga in the below-ground stages was determined separately in an agar-gel assay and a pot trial. The addition of Striga-free control plots facilitated the calculation of the relative yield loss, which represents the result of resistance and tolerance combined. Correlation analysis indirectly demonstrated that both resistance and tolerance are important yield determining traits under Striga infestation. Tolerance was relatively more important under low Striga infestation levels, whereas resistance was relatively more important at high infestation levels. With respect to resistance, both the area under the Striga number progress curve (ASNPC) and maximum above-ground Striga number (NSmax) turned out to be discriminative and consistent selection measures. Both measures also corresponded well with the expression of resistance during below-ground stages of the parasite. It proved more difficult to arrive at a satisfactory measure for tolerance. Inclusion of Striga-free plots is an essential step for the determination of tolerance, but in itself not sufficient. It provides a basis for the determination of the relative yield loss, which then needs to be corrected for differences in infection level resulting from genotypic differences in resistance. A linear correction for infection level disregards the density dependency of the relative yield loss function. It is expected that clarification of the relation between Striga infection level and yield loss, provides a solid basis for the development of unambiguous tolerance measures in the field. This will enable the breeder to select for resistance and tolerance separately, which is likely to result in the optimum combination of both defence mechanisms