All my trees are standard bareroot trees or benchgrafts or trees I grafted myself. The largest would be like a Cummins nursery grade 1 tree.
There is fungicide resistance on lettuce so I don’t think you need fruit to be present to have resistance develop on fruit trees. It looks like any time you have a large population of fungus being exposed to a single mechanism of action fungicide you have a risk of developing resistance. If you expose the fungus to low levels of the fungicide either thru low rates or poor spray coverage the risk goes up.
A nursery is really a farm only it raises trees as a crop instead of fruit. So for fungicides cost per acre or cost per acre for a certain level of effectiveness probably drives choices in buying fungicides as much as anything else. Oplea posted about costs per acre here.
It looks like single MOA fungicides are cheaper than Captan at least in some cases. So I would expect nurseries to use them to lower their costs.
The nurseries are not isolated either. Budwood and rootstocks come into the nurseries from the outside. Now some nurseries probably grow all of their own budwood but few grow their own rootstocks. The rootstock producers are large specialized nurseries themselves and they need to spray their crops as cheaply as possible. In many cases you will have large numbers of small trees planted at high densities which are being sprayed with single MOA fungicides and I think resistance developing there is a distinct possibility.
Now in practice I don’t know how high the risk is and I don’t know what steps the nurseries are taking to prevent resistance from occurring. I also don’t know if they have a testing program looking at resistance in their fields. I know many nurseries have programs to prevent latent viruses from getting into their stocks but for fungicide resistant fungus I have no idea.
I used to think of my small orchard in an area where apples are not grown commercially as being rather isolated. And that coupled with the small size of my orchard shielded me to a large degree. But I am starting to realize that I am connected to the nurseries and rootstock growers and that means I am not as well protected from the issue as I would like.