Citrus gummosis is the most destructive and serious fungal disease of citrus in the world. This study was conducted to characterize occurrence, distribution and casual agents of citrus gummosis in farms and nurseries in Oman. A survey over 2009 to 2011 which covered 17 wilayats and 166 farms showed that citrus gummosis occurs in 15% of acid lime farms and in 35% of sweet lime farms. Gummosis symptoms were also observed on mandarin, grapefruit, Tahiti lime, lemon and orange. A comprehensive survey in Samael in 2010 and 2011 showed association of citrus gummosis with "Round" and "Boo Reqab" cultivars of sweet lime. The disease was observed in 6 out of 9 villages in Samael, with Wadi Al-Saijani and Faghrah villages showing the highest incidence of the disease. Survey in commercial nurseries showed that citrus gummosis occurs on acid lime seedlings originating from Oman, Tahiti lime imported from Lebanon, sweet lime from Oman and Lebanon, orange man from Syria and Egypt and mandarin from Syria. This provides evidence that seedlings imported from abroad could act as potential sources of citrus gummosis into Oman. Isolations from bark, root and soil samples obtained from diseased citrus trees and seedlings from farms and nurseries showed association of 30 fungal and oomycete species with citrus gummosis, with Lasiodiplodia theobromae and Fusarium solani being the most common. When these fungi and oomycetes were inoculated on acid lime and sweet lime seedlings, only L. theobromae, Pythium vexans, Neoscytalidium dimidiatum and F. solani reproduced citrus gummosis symptoms, with L. theobromae being the most aggressive. This study reports for the first time the presence of 13 new fungi and oomycetes to Oman. It also appears to be the first study to report association of P. vexans with gummosis symptoms in acid lime. The study recommends applying extension programs on citrculture for farmers in the most affected areas, applying strict quarantine measures to avoid introduction of new disease problems into Oman, and doing further studies on the casual agents of gummosis in other citrus species in the country.