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.
Many leaves may have spot or blotch like discolourations due to environmental factors or through past diseases that were halted by the plant's immune response. Leaf spot symptoms should be taken with a pinch of salt towards the end of summer as they may be the result of the plant starting to drop its leaves.