Citrus decline with complex symptoms was recently observed in Kerman province, reducing the annual yield of citrus fruits. Agents that cause citrus decline include fungi, bacteria, viruses, nematodes, and abiotic factors. One of the most important symptoms of citrus decline is branch dieback. In this study, the fungal agent responsible for dieback was identified. To achieve this goal, sampling was conducted in citrus orchards with symptoms of twig dieback and twig canker in the summer of 2020. Samples were collected from different areas of the citrus growing region of Kerman province, including Bam, Rigan, Narmashir, Fahraj, Shahdad, and Orzueeh. Fungi were isolated from diseased branches on nutrient culture medium. The resulting fungi were identified using morphological characteristics. The following fungi were isolated from diseased plants: Alternaria spp., Nattrassia mangiferae, Ulocladium spp., Bipolaris spp., Fusarium spp., Phoma sp., Rhizoctonia sp., Paecilomyces spp., Chaetomium spp., Acremonium spp., Stemphylium sp., and Cladosporium spp. Notably, Alternaria spp. exhibited the highest frequency and widest distribution among them, making it the most prevalent fungus throughout all citrus-growing regions of Kerman province. N. mangiferae was the second most frequently isolated fungus. Pathogenicity tests were performed by artificially inoculating two-year-old lemon trees under controlled conditions. All fungal genera except Chaetomium sp. and Cladosporium spp. were virulent. N. mangiferae was the most virulent fungus and caused the most severe cankers six weeks after inoculation. It also had the greatest discoloration compared to other fungi in these tests