Thats usually black cherry aphids, uncurl the leaves and look for aphids.

They are hard to reach with sprays. If you only have a little bit spray insecticidal soap and use one hand to open up the leaves while you spray in. Apply a band of tanglefoot to the trunk to keep the ants off the tree, if there are no ants to move the aphids around they will fade. If you don't apply the tanglefoot they will keep coming back. I had major problems for about five years with these guys, until I got smart with the tanglefoot.

Scott

Thank you so much! I will find out what tanglefoot is and get some and put it around the trunk. I am mixing up a batch of soap spray right now.

Put a band of tanglefoot all the way around the trunk, say 1' off the ground. Make sure there is no other way for ants to get on the tree. To avoid hurting the trunk first put down duct tape and then put the tanglefoot on the duct tape. Make sure there are no tunnels under the tape, check back and see if ants found a way around or under and patch the leak. Collectively ants can be smart, but with a little work we can out-smart them.

Scott