Here dormant oil is used to kill mites and other insects that are harbored in the bark and buds of plants in early spring and not as a fungicide. I have never seen that suggestion on the containers that I have purchased. Which is why I worded my question as I did. You do realize that those leaves will be dropping off the plant when it freezes, which is another thing you should be thinking of. Other than removing the leaves when they drop not much you can do this year that will prevent the problem next year. Many things can cause dark splotches on leaves and not all are fungus. This year a couple of mine were damaged because after a cool summer there was an overnight warming. The edges of the leaves cooked. The center part of the leaves are fine but the edges are dry and point upward. Hail, very large drops of rain, wind driven debris among other things can cause damage that appears to be a fungus but is not. Most peonies will show some type of damage on the at this time of the year unless you live in the Southern Hemisphere.