The mother plant grown in an acidic soil & less hard minerals tends to have thinner and softer tissue, thus prone to fungal infections (BS, mildew). I notice that at higher pH, the stems are thicker (big like a pencil), thus there's a good storage of alkaline minerals in such stems to sprout roots.

Still remember when I was pregnant at 40 year old, the doc. ordered me to drink 3 glasses of cow milk (for minerals like calcium, magnesium, potassium). I was lactose intolerant and took L. Acidophilus probiotics while gulping down 3 glasses of milk per day. Same with plants, rootings are more successful if they are taken from a mother-bush well-fortified with alkaline minerals.

There's a juried scientific publication about the persistence

of Downey Mildew in roses, even in plants where it appears to have been cured.

That would argue for taking cuttings from the part of a rose that are very soft 'wood' where the diseases hasn't been transmitted yet.