Dieback is one of the most common diseases of mango in different parts of the world, including Oman. Despite efforts to control the disease using different fungicides, disease incidence is still high. This study causal agents of the disease in two farms in Oman and the degree of sensitivity of fungal isolates to four common fungicides. The survey showed association of dieback symptoms with most mango cultivars in the farms, with some cultivars having higher disease incidence. Isolations from 40 mango trees developing dieback symptoms revealed association of Lasiodiplodia spp. with 95% of the trees. and Fusarium spp. were isolated from only 10% and 8% of the trees, respectively. Further identification of 67 Lasiodiplodia isolates using sequences of the internal transcribed spacer region of the ribosomal RNA gene (ITS rRNA) and sequenceof the translation elongation factor gene (EF) revealed that all isolates areLasiodiplodiatheobromae. Pathogenicity test of two randomly selected isolates of L. theobromae on 'Amerbally' and 'Omani' mango cultivars showed that the four-month old seedlings started showing dieback symptoms within two weeks of noculation. Testing sensitivity of 28 randomly selected L. theobromae isolates to four common fungicides showed that the EC50 levels of the isolates were in the range of 0.01-8.75. (avg. 0.54 mg 1) for iprodione, er 0.1-242.8 (111.6 mg 1!) for copper oxychloride, 40.3-738.1 (avg. 229.3 mg 1!) for copper hydroxide and 0.1 to over 1000 (avg. > 1000 mg 1!) for thiophanate-methyl. The current study shows the association of L. curi theobromae with dieback symptoms and the development of resistance to some fungicides, especially thiophanate-methyl, Development of resistance to fungicides could be one of the main reasons behind the reduction in the efficacy of managing dieback in the studied farms.