If you are using a surface-protectant fungicide, the sufaces must be completely coated. Traditional instructions are to spray enough to get some dripping. If you are using a systemic such as Banner or Bayer, it's probably enough to get some fungicide on every leaflet.

Evie,

This is a great question, I've often wondered the same. For instance, I tend to spray to the point where leaves have chemical dripping off of them or are shinny when I use Dithane, but when I use Messenger I only apply enough to lightly coat the leaves. Conversely, I drench my roses when I'm using the Ortho products with the hose-end sprayer. I've been doing this after some trial and error. Hopefully some others will chime in and share her/his experience.

Adrian.

I have been wondering this as well. I use the Bayer and I spray the tops really heavily until they are dripping. I try to do the undersides equally well but it is a bit daunting.

Now my collection is small and young. But some of the minis are tough because they are so closely settled (whatever that is called) so how can you possibly hope to get every leaf on every leaflet?

I don't have any large shrubs but that must be daunting too to try and seriously get every leaf. I am only going to live another 60 years or so and I can't spend 10 of them spraying.

If you like the idea of minimizing the use of fungicides, this is an excellent point to explore. In my garden, in the middle of Michigan, I get essentially 100% control of blackspot with a biweekly, very light spraying of Banner Maxx (actually I use a cheaper generic brand, available in the big box stores, Infuse). I make no attempt to spray the bottom surface of the leaves. I do add just enough liquid dish detergent to get an even film on the leaf surface but I do not spray to the point of runoff. I can spray 50 roses with 1 gallon of spray. I read often that the average rose pro treats about 16 roses per gallon of spray and very carefully sprays the under and upper surface of each leaf.
Blackspot spores almost never stick to the bottom surface of leaves and start an infection. I suspect the reason why the bottom is sprayed is that absorbtion of the spray will be especially efficient because the leaf pores are on the bottom in roses and the more systemic you get inside the leaf, the better the control of the disease. I suspect this heavier spraying might be necessary where disease pressure is high, but the only way to know for sure is to see what works in your garden.