There are 100s of leaf spot diseases, many of which are not a danger to the plant. The main causes of leaf spot disease are fungi but they can also be caused by bacteria, viruses, and environmental damage. Determining which of these is the cause can be very difficult, however, fungal leaf spots can often be distinguished by small black bodies in the spots. Leaf spot disease can cause leaf material to die and drop out causing a symptom called shot-hole.
Varies between species but often high humidity and poor air circulation contribute to the spread of the diseases.