Ceraocystis wilt of Acacia manggium in Sabah : understanding the disease and reducing its impact

Abstract

A canker and wilt disease caused by the fungal pathogen Ceratocystis manginecans has devastated Acacia mangium plantations in Southeast Asia. The disease develops when the pathogen enters wounds of the stems of the trees. The wounds are caused by wind damage, branch pruning, animal feeding and borer infestation. Various insects, including nitidulid beetles and scolytine wood borers, have been shown to be closely associated with the development of this disease, although a vector relationship has not been established. The disease has never been found on the roots of trees and isolations from soil in heavily infested plantations have failed to yield cultures of C. manginecans. Research was initiated in 2012 has focused on developing an inoculation protocol to select A. mangium with tolerance to infection by C. manginecans. In order to achieve this, an isolate of the pathogen identified using DNA sequencing technology and having a high level of aggressiveness was selected. Preliminary trials showed that inoculations need to be conducted on established trees with well- developed vascular tissues, ideally one-year-old, and that tests on small plants are meaningless. The ideal inoculation technique involved inserting a single plug of C. manginecans mycelia into wounds made on the stems of one-year-old trees and monitoring the results during the subsequent 12 months, at which time most trees would have died. Over a period of approximately eight years, inoculations were performed on 6 000 such trees representing 140 A. mangium families. The small number of surviving trees was retained by grafting and subsequent vegetative propagation. Re-inoculation of these putatively tolerant trees has led to the identification of approximately 50 clones having high levels of disease tolerance. These trees can now be used to establish seed orchards and for hybridisation with Acacia auriculiformis, which is known to be substantially less susceptible to C. manginecans than A. mangium. The results suggest that it may be possible to pursue plantation forestry utilising A. mangium, most likely as a hybrid partner with A. auriculiformis.