I'm not an expert, but I don't think plums gets peach leaf curl. My guess is aphids, but again, I've only been growing fruit trees for a few years. Here are some photos for plum diseases, see if anything rings a bell:

http://www.ipm.ucdavis.edu/PMG/GARDEN/FRUIT/plumsprunes.html

Carla in Sac

Murky:

That is not peach leaf curl, and appears to be insect related as Carla suggests. Have you closely examined the undersides of the leaves? I would not be surprised to find something like aphids or mites. It may be necessary to spray the trees at some point, but it would depend on what is causing the problem. The damage does not look like any disease I am familiar with.

Don Yellman, Great Falls, VA

It's not peach leaf curl.

It looks like leaf curl plum aphid. Here is a link for info about the insect and its management. http://www.ipm.ucdavis.edu/PMG/r611301811.html
The picture is of a winged adult. Most adults are not winged at the time you first notice the curling damage. See link below for what the leaf curl of this pest looks like.

I have the critters on a couple of my pluots. I have seen the adult aphids when I uncurled the leaf. They are typical aphid green color as opposed to the brown color in the UC Davis photo in the above link. They are troublesome to control as the aphid stay inside the curl and sprays won't reach inside the tight curl. UC Davis IPM is saying moderate control with dormant oil for the eggs.

The pest does not prevent a crop on my pluots although they do reduce the sugar content in the fruit.

I have achieved some control by releasing lady bugs directly on the tree. Wet the tree down with the hose before you release the lady bugs. They are thirsty and hungry when released. They'll hang around the tree doing their job if there's food and water conveniently right there. Release at dusk because they are not as apt to fly away in nightime darkness.

Tony

Here is a link that might be useful: leaf curl plum aphid UC Davis

Thank you Tony and others. That's a big relief. I was originally dismissive of the aphids idea because I've seen several types before and I didn't see anything familiar on the tree.

But I'd never ID'd the winged ones before. The picture from but I think I may have seen those winged bastards. I'll look more closely today.

I don't get much of a chance to inspect the tree during the week and its been pretty much raining every day since I first noticed the curling.

The tree is not that big. I could unroll every leaf while spraying if I had too, as long as its something like aphids where I'm not worried about spreading some pathogen from leaf to leaf as I touch them.

I had aphids on my plum last year and they really did weaken it, so this year in addition to keeping an eye out for them, I am putting tanglefoot around the trunks of my trees so at least the ants can't encourage the aphids and discourage the predators.
Carla in Sac

I made the positive ID on the aphids yesterday. Don't I feel silly! Thanks everyone.

I did consider aphids when I first saw the leaf damage, but I couldn't see them. In the past the aphids themselves had been much more visible long before there was that much obviously visible damage to the leaves of the tree in question.

In my defense, it was gloomy and rainy out the first time I looked and the damned critters are exactly the color of the leaves and are rolled up inside.

I mixed a tablespoon of pure neem oil with a tablespoon of some simple organic dish soap in 2 quarts of water and drenched the tree from above and below the best I could.

I didn't uncurl the leaves as I went. I tried to get liquid in there as best I could but I'm sure I missed some. I have seen a single ladybug in my yard on my cherry tree.

BTW. Neem stinks! (literally, I'm talking about the smell). It took me a little while to place the slightly familiar smell until I realized that it smells like one of the most offensive odor component durian.