Abstract
Quantitative disease resistance (QDR) is controlled by multiple quantitative trait loci (QTLs)/gene(s) and thus confers broad-spectrum or non-race-specific resistance. QDR is more effective at adult plant stages, providing partial and usually more durable resistance than race-specific genes. QDR against various disease-causing pathogens has been observed in many crop plants, but less is known about loci/gene(s) responsible for quantitative resistance, particularly the molecular mechanisms controlling variation in quantitative resistance. Understanding QDR is essential and is of practical interest to many plant biologists and breeders in order to improve crops to cope with evolving plant pathogens. QDR has diverse biological and molecular bases as revealed by cloning of QDR loci and identification of the candidate gene(s) underlying QDR loci. Recent studies on cloning of QDR loci in a few crop and model plants have paved the way to understanding of various underlying molecular mechanisms of quantitative resistance and possible utilization for crop improvement. This chapter focuses on various models explaining QDR, QDR dissections, cloned QDR genes against various pathogens in different plant species and their molecular mechanisms of resistance, and new tools to find and dissect more QDR loci.
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