Dear all,
I've experienced that roses are really funny in their mutations : perhaps the ones you bred were particulary sensible to mildew or to blackspot. I have one plant onto the northen facade of the house, no air and pretty no sun : this is the larger one photographed. It has some mildew but exactly like other roses nearby (Old Blush, Baron G. de l'Ain, Princesse de monaco...).
On the other hand some Zéphirine drouin cuttings (the original plant in the northen facade was bred by graft) in more ventilated situation never suffered of anything. Please consider that they are close to some other plants declared by breeders totally not subjected to mildew i.e. Leonardo da Vinci, which is instead my worst mildew nightmare in my garden (I explanted it this fall and moved on the top of the hill finger crossed)...so sometimes there can be sports of the roses more or less resistants. The original Zèphirine drouin was purchased in Tuscany from "Barni" and other Tuscany friends confirm the strenght of this rose. Perhaps it worth a trip in Italy ..... Monferrato area where I live is the rose paradise. they start to bloom in April, and (even the old varieties) continues until december in a May like bloom flushes. If you planned a trip you would be all very welcomed