Khaya senegalensis Trunk Collar and Root Rot

Khaya senegalensis or Senegal Mahogany (Family: Meliaceae) is regularly seen along the roadside of Singapore's roads. I would say that new plantings of K. senegalensis by the National Parks Board stopped by around 2010. To digress, I have not seen new plantings of another Mahogany, Swietenia macrophylla (Honduras Mahogany) since... ever. And to digress further, those two are introduced species and the only native Mahogany species that I know of are Xylocarpus rumphii, X. moluccensis and X. granatum, which are coastal/rocky beach or back mangrove trees.

Decay caused by fungus in the trunk collar and root (also called Charcoal stump rot) was observed in K. senegalensis in Singapore since the 1980s. Although the disease could spread across a whole stretch of trees, it does not seem to be as devestating as the Fusarium Wilt Disease in Angsana. Infected K. senegalensis would decline slowly over a period of 1 to 2 years. One of the signs of the causal fungi, Kretzschmaria zonata (synonym: Ustulina zonata) is inconspicuous fruiting bodies on the surface roots.

Symptoms are as follows:

• Trees are first noted to be partially defoliated. With each new growth, the leaves are smaller and of lighter green showing progressive decline over the years.
• At the tree collar under the cracked bark, wood tissue is discoloured which signifies decay. White fungal growth may be noted under the bark fanning out over the collar area.
• Tar patches are noted on diseased roots which also show a network of black mycelia over the decayed wood. The tar patches are the initiation of the fungal fruiting bodies. These are low lying crusty carbonaceous layars varying in size 5-20mm thick, buried in the soil and often unnoticed among the cover of grass. Fungal spores are present in chambers within the black lumps.
• The pathogen penetrates deep into the centre of large trees causing a heart wood rot, turning them to dry brittle wood. Such decay can lead to unsuspected tree failure.
• Disease spread to near by K. senegalensis along the tree row via root contact with diseased roots and fruiting bodies.

What to do:

• Remove diseased trees including excavation of the entire stump and as much of the roots as possible for incineration. Also search and remove all black fruiting bodies from the infected area.
• Plant holes may be fumigated to rid the fungi before planting.
• Alternatively fungicide drenches with benomyl or thiophanate-methyl may be applied as preventive measures to protect surrounding "healthy" trees.

Reference - The Singapore Plant Health Guide by the Agri-Food & Veterinary Authority of Singapore

K. senegalensis wood colour ranges from a very pale pink to a deeper reddish brown. Color tends to darken with age. Shown here with the wood of other tree species stacked on top.

 Even large wounds close up completely very relatively fast. Nice dome shape.

 Maybe I was pre-condition to spot the "trunk collar and root rot" but this wound wood roll does seem to be discoloured.

 But tree does not seem to exhibit partial defoliation or dieback, so it should be alright.