ELI5.

In plant/pathogen interactions there are two main groups of resistance.

Qualitative- A few genes that inhibit the pathogen almost completely. The pathogen is classified by these resistant genes into races. This is the type of genetics most of you learned with Punnet Squares in school.

Quantitative- what this paper is about. Many genes act in an complex interlinked fashion to control a disease. These genes can be located in groups or scattered around the genome. This has made creating usable molecular markers extremely difficult for breedings efforts. Traditionally you figure out molecular markers by running a few thousand of them and then using statistical models to correlate where the observed phenotype is linked to in the genome. It works really well for 1-2 gene traits. Not very well for 10+ gene traits.

This is basically the researchers saying "fuck it" we'll just sequence the entire genomes to figure this one out.