1. Gummosis on Acacia decurrens, an invasive tree species, that got established in Merapi Volcano National Park (MVNP) after the eruption of Mount Merapi in 2010 was studied to i) identify the causal organism of the disease, ii) analyze disease symptoms, iii) understand the spatio-temporal distribution of gummosis in the tree population and iv) examine how the disease affects the anatomy of tree wood. 2. Pathological, morphological and molecular studies were used in this studies. 3. Ceratocystis fimbriata was proved to be the causal organism of the disease. The disease spread was probably aided by the ambrosia beetle (Euwallacea sp.) which bores holes on the stem. 4. The disease is noted to spread from the base of the trees, where the ambrosia beetle bores holes first, to the upper part. 5. The number of parenchyma cells in the infected stem was significantly more than in the healthy stem which apparently facilitated water and nutrition transport within the tree helping it to grow normally despite serious gummosis. 6. The management of invasion by A. decurrens in the MVNP area poses a serious challenge due its success as an invader in the volcano impacted area and the threat of the gummosis pathogen spreading to other species both of which will affect the regeneration and establishment of native species and recuperation of the ecosystem.