Vine maples are not overly prone to PM but where they are sited and how they are being cared for may contribute to the problem. These will easily tolerate full sun in our climate and the more sun they receive, the less chance of PM. My Pacific Fire in full sun never showed a trace of PM. Good air circulation helps - mine was at a beach front property and while not in direct wind, did receive a lot of breezy air :-)

Aconitifolium is not a fast grower under the best of conditions. And growing it in a container is a natural dwarfing mechanism so I would not be surprised at what you consider "sluggish" growth. It sounds like you are supplying it with the conditions it needs. btw, your fertilizer regime is exactly the same as mine, although I use the Dyna Gro product infrequently as the Osmocote seems to supply pretty much all that is required.

Thanks for the response gardengal. Is there something I can spray on the vine maple foliage to clear it up? I have access to copper spray and neem, although I know neem causes leaf drop in Japanese Maples. I could also do like a milk and water spray or a baking soda spray.

Unrelated third question - are there any cultural practices that can help maples to show great fall color in containers? Last year my aconitifolium foliage just turned brown and dropped and I'd love to see the beautiful color the cultivar is known for this fall.

Anything other than water on the JM foliage can cause leaf drop. These are extremely sensitive to anything sprayed on their foliage.....so I don't :-) I doubt the vine maples will have as strong a reaction but at this point in the season, there is nothing you can do. The leaves will begin dropping shortly and you can't "clean up" PM damaged foliage.....you can only attempt to prevent it from happening to begin with and it is too late for that.

Fall coloring on any tree is a factor of many conditions, mostly due to climate and watering. Colder evening temperatures and warm, sunny days as we move into fall should help. And if you start backing off of the watering a bit, that should help. A bit of drought stress (not too much - that can cause early leaf drop) combined with cooler temps (especially at night) helps to condense the sugars in the leaf tissue fluids and stimulates the colored pigmentation (anthocyanins, carotenoides, etc) at the expense of the chlorophyll.

The aconitifolium must have heard me call it sluggish, I noticed while watering it today that it has buds breaking throughout the canopy, so I'll get a second flush this year after all!