The problem:

Acacia mangium root rot is the result of an organism (usually a fungus) that penetrates the defences of the root, then killing and digesting the tissues of that root – a pathogenic, parasitic interaction.

Because it affects the tree below-ground, root rot is often first detected when trees begin to show above-ground symptoms such as yellowing or thinning of the crown. A slide show presenting the above and below ground symptoms of root rot can be viewed here. Tree death appears to occur quite rapidly after the onset of above-ground symptoms. 

Currently we hypothesise that this is because the visible above-ground symptoms are a product of acute water stress caused by the direct or indirect action of the pathogen cutting off the water supply at the base of the trunk. The first tissues to be killed and digested appear to be the phloem / cambium tissues close to the surface of the root that carry sugars, produced by photosynthesis, from the leaves down to feed the roots. The fungus either directly (by filling or degrading the water carrying xylem tissues of the root) or indirectly (by starving these tissues of sugars) compromises the capacity of the affected root to provide water to the leaves. 

Even so, the tree can remain alive as long as some of its roots are able to transport water to fuel photosynthesis. The effect of reduced water supply from individual roots killed by root rot are likely to be distributed throughout the crown in ‘hardwood’ trees because ‘pores’ (pit channels) in the xylem vessels allow water to diffuse around the tree as it is drawn up through the waterconducting tissues. 

However all the roots depend on the sap flow down through the phloem of the tree trunk. Should the pathogen reach the base of the trunk, the “root collar” where the roots emerge, it can grow through the cambium of the trunk completely encircling the tree, internally ‘ring-barking’ it and cutting off sap flow to, and reducing water flow from, all the roots. This soon leads to tree death. 

As trees die, gaps form and expand in the plantation, apparently as the infection spreads from tree to tree. Fungal fruiting bodies can be found in association with root rot in Acacia mangium, once planted in the field, appears most commonly caused by basidiomycete fungi of the Polyporales.