Iran is considered a region of the world with low forest cover. Compared to other temperate areas with better soil conditions, it is difficult to achieve afforestation on a large scale and as a consequence, most of the afforestation is conducted with non-native species. Conifer afforestation is one of the forest development top priorities of Iranian forestry department in aiming at optimal use of marginal lands with less fertile and rockier and arid soils. One of the most important impacting factors that hinder the establishment of conifer plantations after afforestation is their poor growth and establishment followed by gradual decline of trees in some regions. There are a number of investigations carried out on the causes of death of conifer seedlings in nurseries, however very little work has been done on identifying causal agents of decline of coniferous trees. The objective of this study was to investigate the etiology of conifer decline with emphasize on identifying the role of biotic (fungal PATHOGENS) or abiotic (edaphic) factors. The geographical area of this research was Fars province, Iran including seven selected sites across the province as follows: Fasa, Arsanjan, Cheshmeh Abolmahdi, Zarghan, Shiraz, and Enghelab National Park (control) with the main focus on Cheshmeh Abolmahdi plantation as the major research site. During many field visits of research sites, roots/crowns of suspected declining trees and their surrounding soil were sampled by digging out soil profiles and suspected tissues were cultured on general (PDA), semi-selective (acidified PDA), and selective (PARP-CMA) media with/without surface sterilization after they were thoroughly washed under running tap water. In preliminary soil analysis of research and control sites general soil characteristics (texture, pH, and salinity) were measured. Isolated fungi were purified by single spore and/or hyphal tip techniques and identified using authenticate taxonomic keys/monographs. Pathogenicity of isolated fungi was investigated by inoculating young seedlings under greenhouse conditions after providing them with condition conducive for infection. Isolated fungi were identified as per following: Fusarium solani (nine isolates), Fusarium sambucinum, Fusarium avenaceum, Fusarium graminearum, Fusarium subglutinans, Fusarium crookwellens, Rhizoctonia solani, Binucleate Rhizoctonia solani like fungus, and Pythium okanoganens (one isolate of each). Two isolates of F. solani, one isolate of each of F. sambucinum, R. solani and P. okanoganens were pathogen on pine and/or cypress seedlings. Among these isolates the latter was relatively highly aggressive on pine seedlings whereas the others were not as much so. The role of soil texture (including compaction and hard pan layer), pH and salinity as possible contributing factors in gradual decline of conifer trees in research sites is discussed.