PP261, a 2-page illustrated identification sheet by Megan M. Dewdney and Jamie D. Yates, can be used to identify four fungal diseases in citrus: Alternaria Brown Spot, Citrus Scab, Greasy Spot and Melanose. Published by the UF Department of Plant Pathology, March 2009.
http://edis.ifas.ufl.edu/PP261