Characterization and evaluation of fungicide resistance among Lasiodiplodia theobromae isolates associated with mango dieback in Oman

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 in some farms in the country. This study involved in-depth analysis of the 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 the association of dieback symptoms with 100% of mango cultivars in the first farm and 47% of mango cultivars in the second farm, with some cultivars having higher disease incidence (e.g. cvs Rothana and Zafran). Isolations from 40 mango trees showing dieback symptoms revealed the association of Lasiodiplodia spp. with 95% of the trees. Alternaria 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 sequence of the translation elongation factor gene (TEF) revealed that all isolates belong to Lasiodiplodia theobromae. Pathogenicity test of two randomly selected isolates of L. theobromae on cvs Amerbally and Omani showed that inoculated seedlings started showing dieback symptoms within two weeks post inoculation. 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 l^-1) for iprodione, 0.1-242.8 (111.6 mg l^-1) for copper oxychloride, 40.3-738.1 (avg. 229.3 mg l^-1) for copper hydroxide and 0.1 to over 1000 (avg. > 1000 mg l^-1) for thiophanate-methyl. The current study showed 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.