I just received an email today with this very exclamation. It can be scary when plants take a turn for the worse, often for no apparent reason. The first step in troubleshooting is usually checking for over or under irrigation. After that step, things can get tricky. One of the best resources for pest and disease problems afflicting ornamental plants is the UC Integrated Pest Management (IPM) database. Here you will find overviews of common pests, diseases and environmental factors of many common landscape plants.
Below is an example of problems that can afflict Strawberry Tree, which was what my client was concerned about.