I can't make out the details of the problem from the photo you've attached but from past experience the moxt common problem of the two is the borers. You will likely see some exit holes on the trunk and larger branches of the tree where adults have emerged. These may however be plugged up and covered with the oozing sap. We recommend deep and infrequent watering to keep the trees healthy and no so susceptible to insect and disease damage. We would also recommend winter watering, especially with riparian species such as chokecherry. If you would like more information on watering techniques, please contact the Natrona County Extension office or Master Gardeners at 235-9400.