Symptoms - Thousands of species of fungi, bacteria and viruses cause leaf spots. There are many instances where spots on leaves are just one symptom of a disease which is causing more serious damage elsewhere.
Also be aware that other causes can lead to spots on leaves, including misdirected chemical sprays and air-borne pollutants. If the spots are 'uni-directional' and cover a large range of plants this is likely the cause. In late summer and autumn normal seasonal changes to foliage can be misleading, and some plants such as hydrangeas have spots which are entirely normal.