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.
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Plant and Insect Diagnostic Clinic
327 Bessey Hall
Iowa State University
Ames, IA 50011
(515) 294-0581