Joseph Lofthouse wrote:
... recommends starting a plant breeding project by culling plants that are immune, and saving seeds from those that are susceptible. The idea being that immunity due to singe gene interactions is very susceptible to breaking down long term. And that tolerance that is due to dozens or hundreds of genes is much more resilient in the long term.
Just adding some additional food for thought. I guess I would not cull plants that were immune, but I agree that one would want to keep a bit of everything that they thought might be valuable and maintain diversity within their stocks. That some of the genes that confer immunity are indeed single genes and prone to break-down over time is well founded, yet there is some immunity (mostly in the realm of 'non-host immunity') for which increasing evidence points to a diversity of genes that underlie the trait. Tolerance has been the subject of much debate, but I can agree with this discussion and definition:
"A definition that would be limited to tolerance but include its broadest
scope should be available. Tolerance, like other resistance, is a relative concept
and may occur in varying amounts. It may also occur in combination
with other resistance.
Tolerance may be defined as that capacity of a cultivar
resulting in less yield or quality loss relative to disease severity or pathogen
development when compared with other cultivars or crops. This definition encompasses
any degree of tolerance in combination with any degree of other
resistance or lack of resistance in respect to the comparison, thus fitting the
broad conceptual base. " --John Schafer "Tolerance to Plant Disease" (1971) Annual Review of Phytopathology
The good outcome of the high diversity Joseph is selecting for is that there is probably mixtures of different categories of resistance -- even if one of many plants was resistant/immune due to a single, more-precarious gene, the vast amount of resistance mechanism at work in the *population* would almost guarantee long term success as noted.
For a more in-depth discussion on the issue, an open-access paper entitled "Plant Innate Immune Response: Qualitative and Quantitative Resistance" --
https://www.tandfonline.com/doi/full/10.1080/07352689.2016.1148980