There is a category of Q which asks for help identifying the cause of a problem, here's three examples:
I think these are an important category for the site, because they're real questons, they're interesting, potentially difficult, and (I like to think) attractive and challenging for experts to answer.
So do does this category need a tag of its own?
The last example is tagged identification which is OK but which is a valuable tag in its own right for tagging "What is this plant?" type questions. And that's a different type of question.
disease would be another possibility but sometimes if we have a failing plant we don't know if it's disease or pest or environment.