The occurrence of gummosis on invasive Acacia decurrens  after Mount Merapi eruption in Yogyakarta, Indonesia

Sri Rahayu¹*, Rahman Gilang Pratama¹, Muhammad Ali Imron¹, Januar Mahmud¹, Widyanto Dwi Nugroho¹

¹Fakultas Kehutanan, Universitas Gadjah Mada, Jl. Agro No. 1 Bulaksumur Yogyakarta 55281, Indonesia ,* Corresponding author’s email: tatarahayu@yahoo.com

Introduction

Large, but infrequent, volcanic eruptions, such as the 2010 eruption of Mount (Gunung ) Merapi in Yogyakarta, Indonesia, caused significant changes in ecosystem composition and vegetation structure in surrounding areas of the volcano (Anon 2012, unpublished data). Similar ecological impacts were observed in Washington, USA when Mount St. Helens erupted in 1980. The eruption of Mount St. Helens produced a complex of disturbance agents, such as pyroclastic flow, debris avalanche, mud flow, ash deposits, blow down, and several other agents which interacted at specific sites exacerbating the degree of damage and delay in recovery (Adams et al., 1980; Dale et al., 1998; Franklin et al., 1985; del Moral 1993). Moreover, combinations of post-disturbance site characteristics influenced the composition of the biota. The pyroclastic flow from Mount Merapi apparently destroyed all flora and fauna that it contacted in the surrounding area of the volcano.

In a period of 2 yrs following the eruption, Acacia decurrens(Wendl.) Willd. (green wattle) was the first tree species to appear, establish, and dominate large areas impacted by the pyroclastic flow. Data collected by Merapi Volcano National Park (MVNP) in 2012 showed that the density of A. decurrens averaged 16167 seedlings ha^-1 and 11814 saplings ha^-1 (Anon 2012, unpublished data). Native to Australia, A. decurrens is introduced and grown in plantations in several countries in the tropics and subtropics. It is an important invasive species especially in Africa and Oceania where it spreads rapidly via seed and root suckers (CABI, 2018). The tree can invade grass lands, roadsides, savannah and riverine habitats developing dense thickets impacting on native biodiversity and obstructing water flow (Boucher 1978). The rapid establishment ofA. decurrens in the MVNP area poses a serious threat since it will affect the recuperation of ecosystems and regeneration of native flora impacted by the volcanic eruption.

A preliminary survey conducted in 2014 by MVNP on the health of A. decurrens stands showed that the ambrosia beetle, bagworm,Ceratocystis sp., Uromycladium falcatarium , andGanoderma sp. occur in the stands. Rahayu et al. (2015) have reported that over 50% of A. decurrens stands in the region showed gummosis and symptoms of stem canker associated withCeratocystis sp. However, these symptoms were not visible on trees within the area around Mount Merapi before eruption. Gummosis ofA. decurrens and Acacia mearnisii caused byCeratocystis fimbriata Ellis and Halsted was recorded from Brazil and South Africa, respectively by Ribeiro et al. (1988) and Morris et al. (1993). The symptoms described onA. mearnsii were similar to those observed on A. decurrensin MVNP. The genus Certocystis includes some well-known pathogens of trees responsible for a wide range of diseases including stem cankers, vascular wilts and root diseases (Kile, 1993) For e.g.,C. fimbriata is known to cause diseases of several vegetable crops, fruit trees and forest trees (CABI, 2014). Another species ofCeratocystis, C. albifundus occur on Acacia mearnsii plantations in southern and eastern Africa causing gum exudation, wood-discoloration, stem cankers, rapid wilting and tree death (Morris et al., 1993; Roux et al. 1999). It is also reported that wounds on trees are a predisposing factor for C . fimbriatasensu lato (s.l .) to cause infection. These wounds can result from wind and hail damage, growth cracks, insect and animal damage as well as activities such as grafting and pruning.

Recent studies have shown that artificially induced wounds on trees are infected by Ceratocystis spp. (Barnes et al, 2003; Roux et al, 2004; Rodas et al, 2008). Success of infection is dependent on a number of physical and environmental factors. For e.g., C. fimbriataspecies complex are able to infect their hosts when viable fungal propagules are deposited onto bark wounds (DeVay et al., 1968). OtherCeratocystis spp., such as C. fagacearum (Bretz) Hunt, can only infect if viable fungal propagules are deposited onto freshly exposed wood of the host (Kuntz and Drake 1957). Temporal factors also affect the success of infection by Ceratocystis spp. For example, Kuntz and Drake (1957) showed that C. fagacearum could not cause infection when wounds were older than 24 h. Climatic factors such as temperature and relative humidity have also been shown to influence germination of spores and infection by Ceratocystis spp. (Cole and Fergus 1956). So, it needs to be examined whether tree wounds caused by insects was one of the reasons for the occurrence of gummosis onA. decurrens in the Mount Merapi region and whether the spread of the disease was assisted by the dominance (monoculture) of A. decurrens trees in the MVNP area.

Against this background, the present study aims to i) identify the causal organism associated with the disease, ii) analyze the disease symptoms and predisposing factors for infection, iii) understand the spatio-temporal distribution of gummosis in the tree population and iv) examine the anatomy of the infected wood. This information would help developing methods to manage A.decurrens and control the spread of the gummosis pathogen to native plants.

Materials and Methods