Bitternut hickory is the most abundant and uniformly distributed hickory species in the north central and northeastern USA. Higher rates of hickory decline or mortality than expected have been recently reported from multiple states in the regions. Although hickory decline historically had been attributed to outbreaks of hickory bark beetle during a drought period, it has been recently described as a “decline disease” for which major determinants have not been identified. In 2007 and 2008, a regional survey was conducted by the U.S. Forest Service to elucidate the major determinants of the recent episodes. The survey found that declining hickories exhibited three types of crown symptoms (e.g. rapid crown decline, top dieback, and slow crown decline) and several types of fungal-associated stem damage (e.g. cankers and galls). However, to date, the potential involvement of fungal agents in hickory decline has not been thoroughly examined. This dissertation presents results of investigations to elucidate fungal agents associated with hickory decline and their relationships with declining trees. Associations of putative pathogenic fungi with major stem damage were determined by collecting samples from geographically dispersed regions and isolating and identifying fungal species by standard methods. Ceratocystis smalleyi and Fusarium solani were two key fungal associates. Their ability to cause cankers on maturing bitternut hickory was confirmed by fulfilling Koch’s postulates under field conditions. Since C. smalleyi appeared to be the predominant fungal pathogen associated with trees exhibiting rapid crown decline, its role in development of the symptom was examined using physiological, anatomical, and histological approaches. A general trend was observed of reduced sap flow velocity in diseased compared to control trees. Of selected xylem properties, the extent of vessel occlusion by tyloses in the outer annual rings was inversely and significantly correlated with sap flow velocity. C. smalleyi was found to induce a set of general defense responses in xylem tissues upon infection but its xylem colonization was locally restricted compared to that of true vascular wilt pathogens. Etiology of rapid crown decline is described using the disease triangle concept and its comparisons with other related patho-systems are discussed.