First, contact your local Cooperative Extension office for help. If the agent needs further assistance, the sample will be sent to a plant disease clinic at your state's land grant university. Most plant disease clinics charge a nominal fee for disease diagnosis. The following link lists the plant disease clinics and soil test laboratories in the US: http://www.apsnet.org/directories/pdfs/Soil%20Labs%20and%20Plant%20Clinics1-05.pdf
For more information on this or other topics related to corn production, contact your state extension corn specialist or your local extension educator/agent.
The following is the link to the corn extension specialists: state extension corn specialist