Neither. A shovel.

Sodium bicarbonate, or better, potassium bicarbonate, will destroy the rose mildew fungus. The originator of the Cornell Formula, Kenneth Horst, has written that the effect is only partly due to the alkaline pH and partly due to specific reaction of the potassium and the bicarbonate ions. Vinegar is not very efficient at killing established mildew, although some people claim scrubbing the surface of the leaves with vinegar works. Neither acid nor base nor a synthetic fungicide will kill established blackspot without killing the leaf at the same time.

Plants have their own method for regulating the pH of their internal liquids and the pH of the soil normally has no effect on plant pH. The color change seen in the flowers of some plants when grown on acidic soil results from a greater availibility of certain metals to the plant, and not from a change in the internal pH.

Thank you, Mike!
A little science in advance of the shovel, but really, I have only ever shovel-pruned the weakest, non-performing plants, and even the ugly, bare legs of Fanton Latour have only ever prompted me to move her where her legs can be hidden by perennials. And I have never ever sprayed for fungii nor insects. Apparently, that's not going to change anytime soon.
Elk.

To HollySprings: When you say a shovel is the cure for fungi, what exactly do you mean? Taking out established roses? That seems pessimistic and dire. Shoveling off the surface layer of soil? Since rose roots grow near the surface I'm wondering how safe this is. What do you suggest? I have distressing blackspot problems that extend even to my disease resistant varieties.