Heritability of clover rot resistance (Sclerotinia spp.) in red clover (Trifolium pratense) populations

European red clover (Trifolium pratense) crops are susceptible to clover rot, a destructive disease caused by Sclerotinia trifoliorum or S. sclerotiorum. No com-pletely resistant cultivars are available. The lack of knowledge on the heritability of clover rot resistance is, among other reasons, responsible for the slow progress of resistance breeding. In this paper we estimated the heritability of clover rot resistance through divergent selection on an experimental European red clover population.
Young plants from our original (O) experimental population were inoculated with a mixture of ascospores from five aggressive isolates. The most resistant plants were selected by mass selection to breed the first generation after selection for resistance (R population). The next year, the most susceptible plants were selected from the O population to breed the first generation after selection for susceptibility (S population), while again the most resistant plants were selected from the R population to breed the second generation after selection for resistance (R² population).In a final experiment, the resistance levels of the four populations were compared: the original population (70.5 %), the first generation after selection for susceptibility (79.2 %) and the first generation after selection for resistance (62.3 %) differed significantly in susceptibility (p < 0.001). The second generation after selection for resistance (60.0 %) was not more resistant than the first generation after selection for resistance. In the first generation of selection the heritability (h²) was on average 0.34. In the second generation of selection, h² was 0.07.
These results indicate that mass selection is not a suitable method to improve clover rot resistance. Instead, family selection or polycross selection should be performed against a strong selection pressure. Future research should evaluate whether a family selection or polycross selection allow a sustained increase in resistance for multiple generations.