I think you are right that changes in field conditions do partially explain the "reduced resistance over time" that we sometimes see.
There may be cases where pathogens evolve in a field after successive years of tomato growing (even if you are practicing crop rotation). But this is hard to separate from the effects of simple buildup of diseases that can occur in a field over time if tomatoes are consistently grown.
Either way, the increasing genetic uniformity that comes with the selection of an OP variety is not the sole reason one might see poorer vigor and/or disease resistance over time.
Quote:
Originally Posted by SeanInVa
I wonder if this is simply a result of the fungi/bacteria themselves adapting, much in the same way we see bacteria that infect humans adapt to overzealous use of anti-biotics, and not so much a genetic trait, or "problem" of OP breeding?
I suppose it would be an interesting experiment to see if the later generations which appear to lose some of their disease resistance when grown by you locally, gain it back if grown in other parts of the county/state/country/world
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