Hazelnut (Corylus avellana L. ) is affected by dieback (DB) and decline (D) diseases causing significant losses to hazelnut production in the Eshkevarat (Guilan province, northern Iran) as the main region for hazelnut production in Iran. Although, causal agents of these disorders have remained uncertain for many years. During 2017-18, results of a survey on DB and D diseases in hazelnut-growing sites (HGS) from Roudsar (54 out of 199 villages) and Amlash (14 out of 124 villages) counties (Eastern Guilan) revealed that mean frequency distributions (%) of DB and D diseases based on the infected HGS in the Roudsar and Amlash counties were equal to 27. 14 and 10. 85%, respectively. DB and D diseases were widespread in the Eshkevarat region, where they occurred in 38. 7-55. 3% of the hazelnut orchards. Mean tree infection (%) ranged from 3. 94 to 28. 3% and 6. 1 to 33. 6% in Amlash and Roudsar counties, respectively. The fungi with different distribution frequencies included Cytospora sp. (33. 60%), Phomopsis sp. (14. 40%), Verticillium dahliae (11. 20%), Lasiodiplodia sp. (16. 80%), Rosellinia necatrix (10. 40%), and Pestalotiopsis sp. (13. 60%), which were isolated and identified based on their morphological and cultural characteristics and were tested for their pathogenicity. Fungal pathogens infected hazelnut trees individually, or in combinations, to cause hazelnut dieback. Most of these fungal pathogens initiate infections at wounds caused by insects, humans, machinery, lightning, wind, and hail.