KNYSNA OAK TREE DISEASE

disease
 

Ambrosia Beetle from diseased Knysna white oak trees

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Knysna Ambrosia beetle: Bark beetle: Xyleborus ..(?)

The beetle carries ambrosia fungus spores and cultivates the fungus in the tunnels it bores. They have a symbiotic relationship – the beetle depends on the fungus as a food source throughout its life cycle and the fungus depends on the beetle to be spread and cultivated. In an infected tree the fungus spreads into the xylem layer and digests it preventing nutrients to flow past the affected areas. In high density invasion the nutrient flow is compromised causing the limb or tree to die.

Treatment: There is no local treatment for ambrosia fungus.
Best practice is to support the tree with additional nutrients (organic or slow release nitrogen) and water to boost the trees feeding and natural immune system and processes.

Water should be applied by drip irrigation around the roots between the inside and outside of the drip line rather than by infrequent flooding sessions where more water washes away than it is able to penetrate or be absorbed into the ground.

Management: Early detection and timely removal of infected limbs or trees is the best preventative measure for protecting unaffected trees. Infected oak trees should not be pruned during October, November and December (southern hemispere) or other warm times of year when the beetle is active (eg Knysna 2013 extended summer). Prune only in cold winter and avoid unnnesesary pruning. Pruning cuts should be sealed to neutralise the attractive sweet smell of freshly cut oak and to prevent the borer from entering the wounded area.

Spread: The ambrosia beetle is attracted to sweet smells like freshly cut oak, fermenting fruit & fruiting fungus etc. They attack dying and stressed trees. Other beetles and insects feed on the ambrosia fungus – it is not known if they are also vectors of the fungus.

Symptoms: Premature defoliation and die back from extremities, 1- 1.5mm holes in the bark with white wood powder at tunnels entrances, blackened bark where high density invasion is present. Sweet smelling black sticky fluid weeping from the tree can also be observed.

Waste management; To supress overland spread of vector/s and the fungus infected tree waste should not be removed from site. Ideally waste should be chipped and burned on site or covered with clear plastic (ends tucked in below the soil line) for 60 days and then burned on site.

Warning: the abundance of beetles means that structural integrity of infected branches is compromised. Bigger branches than normal can be expected to self shed. Arborists must not use infected branches for anchors or rigging crutches.

Conclusion: If not the sole cause the ambrosia beetle is at least the primary cause of the local oak tree disease and deaths.

References: www.wikipedia.org and www.ces.ncsu.edu

Standing oak trees that are dead or stressed are suseptable to being infested by other wood boring beetles also.

Other beetle samples collected from the same infected oak tree are presented below.

Bark beetle: Tomicus (piniperda)

The piniperda is supposed to be found in pines, yet this sample and many others were extracted from the diseased oak tree sample. Might be it is not accurately identified – advise will be appreciated. It breeds in recently dead and dying trees. It is not the cause of the oak tree deaths although it plays a role in speading up the rotting and weakening of limbs process.

This beetle contibutes to significant degradation of timber structure / strength – Climbers beware!!

Size: 3.5mm length; 1.2mm wide
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Powder post beetle: Lyctus

These beetles are to be found in dead wood. They are not the cause of the death of the oak trees. They indicate advanced wood decay.

Their presence indicates that limb strength is compromised – Climbers beware!!

Size: 3.5mm length, 1mm wide
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