A guy in the business has an advantage over the layman, in part because he gets to see so many cases. For example, there have been numerous calls recently about Oak, Maple and other leaves turning brown on th edges and curling up. It's been wet and cool this year so Anthracnose is tearing it up. I've looked at a lot of anthracnose. I could get a call and only hear, I have an Oak...", interrupt and tell them they have Anthracnose, don't worry, goodbye.
Last year it was something else, next year it might be different again. After a while it starts repeating, and all of the sudden you're an expert.
The highlight of your year, then, is if something new comes along. But one's diagnostic process is refined to the point at that time, the solution comes pretty easy.