oak wiltn. A disease of oak trees caused by the fungus Ceratocystis fagacearum that results in discoloration, wilting, and dropping of leaves, and often death of the tree.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.oak? wilt` n. a disease of oaks, characterized by wilting, discoloration, and loss of leaves, caused by a fungus, Ceratocystis fagacearum. [1940?45, Amer.] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.