When possible, include the entire plant, with the root system and surrounding soil. Often what appears to be a leaf problem is really a root-related problem. Enclose the root ball in a plastic bag to keep the soil from touching the leaves. Include enough plant material to show all stages of the disease. When possible, provide several whole plants.
Plant and Insect Diagnostic Clinic
327 Bessey Hall
Iowa State University
Ames, IA 50011
(515) 294-0581