What are the options for verticillium and Fusarium now, btw?
Southern Exposure SE has some ratings on certain heirloom's disease tolerances. I'm sure we have also noticed certain heirloom cultivars showing particular resistances to certain diseases.
I think the trouble is that to know that a variety is definitely resistant, you would need a fairly broad scientific study. A large number of plants, along with controls, with a certain amount of the pernicious agent introduced evenly. Just saying "I noticed my KBX doesnt die of blight when others next to it get it" isn't enough to definitively say that KBX is blight-resistant. Since it is not economically feasible to do these studies for heirloom varieties, people just assume they have no resistance to that disease.
As far as breeding resistance into plants, I
read that for some cucurbits, they found that a specific gene cluster was responsible for increased powdery mildew resistance. If resistance can be attributed to a particular gene or small gene cluster, I'd imagine (not being in any way a geneticist) that one could easily cross that plant with an heirloom and select out a resistant OP with desirable characteristics.
I know genetics can get much messier than that however. Some traits can be spread over many gene groups and can be hard to breed for. Excuse me if I mangle the science, I am an engineer, not a biologist.
If genetic engineering were cheaper and easier, we could just pluck out the gene that gives resistance, plunk it down in our Black Krim, and not worry about getting all the other characteristics all messed up! Unfortunately that science is not attainable for the home grower now.