Walnut (Juglans regia L.) is one of the main nutritional nut crops in many parts of the world. Iran is one of the most important countries in the world regarding walnut production. In recent years, dieback and decline of some of walnut trees has attracted the attention of growers and plant pathologists in Iran. Although the infestation by the Cytospora spp. was accepted as the causal agent of canker, dieback, and decline in walnut trees, the role of the other agents has yet to be well investigated. Field observations were conducted during 2016- 2020 from different provinces in Iran, and black sooty canker was noticed in most declined or declining walnut trees. To determine the causal agent of the observed symptoms, sampling was conducted, and fungal isolates were obtained from symptomatic tissues of walnut trees. The fungus consistently isolated was identified as Neoscytalidium dimidiatum based on morphological and molecular characteristics of the ITS and tef1-α regions. Neoscytalidium dimidiatum was shown to be the cause of the symptoms by pathogenicity testing. The current findings show that N. dimidiatum is a serious and devastating disease in Iran, causing walnut tree decline. تفاصيل المقالة