For mature trees (or young ones), you can hire a certified arborist or licensed tree expert to evaluate them in person. We can help diagnose issues from photos shared through Ask Extension, but sometimes it's hard to make a diagnosis from images alone, or once symptoms become advanced/extensive. Arborists cannot cure or treat every condition, but can help narrow-down a diagnosis and also look for sources of tree stress that might be able to be alleviated. Some arborists have more training or experience with diagnostics versus others that have more pruning and general tree removal safety training, so you can screen candidates based on their familiarity with issues pertaining to the tree type in question. (Say, ornamental cherries, fruit trees, maples, etc.) You can also get a second arborist's opinion before taking any recommended actions, as evaluations by different arborists aren't always in agreement.

Fortunately, a lot of pest and disease management revolves around prevention by keeping tree stress to a minimum (especially since many secondary pests and infections target stressed plants) and by making sure young trees are not planted too deeply (or over-mulched).

Miri