Fungal infection

Why decay happen to living trees?

Robert Hartig, the father of forest pathology, proved that decay did not cause fungi. [2] The answer is all decay of living trees are caused by fungi and bacteria. The fungi have classified into two groups: Basidiomycetes and Ascomycetes.

Basidiomycetes are the most common fungi causing wood decay which grow on living or dead trees. Common fruiting structures such as conks, mushroom. For example, Cerrena unicolor, Inonotus rickii.

Ascomycetes are commonly causing leaf diseases and cankers on urban trees. Those fruiting bodies are less obvious than Basidiomycete. Few Ascomycetes are rotting wood,
For example, Sycamore anthracnose, Kretzschmaria sandvicensis

Sycamore Anthracnose Leaf Damage — leaf damage caused by *Sycamore anthracnose [3]*

Case 1: Tree decay caused by Kretzschmaria sandvicensis

Location of tree: Urban Park
Host: Mature Celtis sinensis 朴樹
Fungus infection area: Trunk base
Status: Mature fruiting bodies

Whole view of the infected Tree *Celtis sinensis*

Fruiting body observed

Observed grey and dark flat lumpy fruiting body attached to root collar. Margin was round, discolored bark around the fruiting body. That kind fungus was identified as Kretzschmaria sandvicensis.

Tree assessment and significance to tree structure

Kretzschmaria species are found throughout the temperate and tropical regions of the world, which is one of the common fungi in Hong Kong.
This fungus is found on Celtis sinensis and Ficus microcarpa in the urban forest. [1]

The host tree was Celtis sinensis located at Urban Park. Visual tree assessment was carried out, mature fruiting bodies of K. sandvicensis was found in trunk base and tree collar region. Decayed wood canker at root collar was observed. As K. sandvicensis is saprotrophic fungus growing on woody tissue, it would induce soft and white rot decay on root collar causing sapwood and heartwood damage. [7] The infected tree would be windthrow and collapse easily.

Sparse foliage and dieback were signs indicating that the rooting system was adversely affected and weakened the ability of transportation of water to leaves. Tree vigor was declining.

Used a mallet to tap around the infected trunk base to determine the internal decay area. A different tone was heard, different wood density or internal decay was suspected. Internal decay would weaken the stability of the structure. Supporting strength and health was going down.

An advanced assessment was conducted to investigate the internal condition of trunk base. Resistograph to drill the tree at 0.3M and 0.6M from ground level. The result showed that a drop of resistance was observed, internal decay was identified.

Last inspection of the infected tree. The smaller infected area at trunk base.

Fruiting bodies of Kretzschmaria spp. at a root collar. The picture was taken in Tsing Yi Park Hong Kong

Mitigation Measure

The health condition of the infected tree was adversely affected and became unstable. This fungus would cause the infected tree to become brittle and easy to windthrow and collapse. Close monitoring is needed and prevent the tree from damage, especially using a mower to cut grass around trunk base and roots zone. It is because further infection may be induced through wounds. Keep the tree healthy is the most important. Remove the fruiting bodies to minimize the spread of the disease to other trees. Failure potential is medium, failure part is the whole trunk, the impact rating is high. The target rating is frequent. The risk is medium. Close monitoring and application of fungicide to inhibiting the growth of fungus were recommended. Reinspection within 6 months was proposed. If found the advanced stage of infection, structural decay more severe, the whole tree should be removed.

Drill 1 *result from resistograph. A significant drop in resistance* from 190mm to 230mm. Soften wood or decay was indicated in this zone.

Drill 2 result from resistograph. Significant drop in resistance from 210mm to 235mm. Soften wood or decay was indicated in this zone.

Case 2: Tree decay caused by Inonotus rickii

Location of tree: Urban Park
Host: Mature Cinnamomum camphora 樟
Fungus infection area: Codominant trunk Joint
Status: Anamorph or asexual stage

Fruiting bodies observed

Fruiting bodies Inonotus rickii grown at the joint of codominant trunk underneath canker was observed. It attached to the substrate firmly. Broadly ellipsoidal yellowish and white edge bodies with rough and thick surface. The species was identified with the field guide “Note on Common Wood Decay Fungi on Urban Trees of Hong Kong”. [1] Discolored bark was observed beneath the fruiting bodies.

*Inonotus rickii grown at the joint of codominant trunk underneath canker was observed.*
*Fruiting bodies of Inonotus rickii comparison last and recent inspection.*

Image Source from [1]
Note on Common Wood Decay Fungi onUrban Trees of Hong Kong

Tree assessment and significance to the tree structure

This fungus has a worldwide distribution extending from tropical to temperate regions which is one of the common fungi in Hong Kong. This fungus is commonly found on broadleaved trees such as Celtis sinensis, Cinnamomum camphora and Sapindus saponaria in rural and urban forest. This is one of white rot fungus causing canker with moderate virulence. It attacks heartwood, sapwood, and cambium on branches and trunks of trees. Wounds induce this fungal intrusion, fruiting bodies could be found on wounds of branches and stem. Eventually, host declines health to death.

During this visual tree assessment, the host tree was mature Cinnamomum camphora located at Urban park. Mature fruiting bodies were observed at the joint of codominant trunks with underneath canker. As the sparser foliage and dieback were observed, the rooting system was adversely affected. The ability of transportation of water to leaves was weakened. Energy reserve was diminishing and tree vigor was declining. The fungus was localized decay and spread over, causing internal decay. Stability of structure was threatened. Split of co-dominant trunk was foreseen. Used a Mallet made of rubber to tap the stem to trunk base around the suspected decay area and listened for tone variations. Not obvious tone difference was investigated when tapping near the infected area. It seemed that the different wood density and extent of internal decay might not be determined.

Mitigation Measure

Fruiting body of Inonotus rickii found on the join of co-dominant trunk. Such kind wood destructive fungi weakened the wood structure and declined tree health. Remove the fruiting bodies, application of fungicide to inhibit the growth of fungus, and close monitoring were proposed. Avoid pruning to minimize the damage of tree reducing the chance of further infection. Cobra cables between codominant trunks should be installed for limiting the tree movement, prevent further splitting or cracks from wind load. Reinspection within 6 months was recommended.
As the site is a recreation area where is used by residents frequently, if observe an advanced stage of infection, structural decay more severe, the whole tree should be removed for sake of public safety.

*Cinnamomum camphora infected by*
*Inonotus rickii recent inspection, lower foliage density than last inspection in May 2018.*

*Cinnamomum camphora infected by*
*Inonotus rickii during the last inspection*

Comparison on the significance of tree structure between these two fungi:

	*Kretzschmaria* *sandvicensis*	*Inonotus* *rickii*
Classification	Ascomycota	Basidiomycota
Common host	Living Celtis Sinesis, Ficus microcarpa	Living Celtis sinensis, Cinnamomum camphora and Sapindus saponaria
Types of wood decay	Soft and white rot decay	Soft and white rot decay
Damage to tissue	Heartwood and sapwood	Cambium, heartwood and sapwood
Structural defect	Root flare, trunk base	Branch and trunk
Significance	Root rot causes whole tree collapse	Internal decay cause part broken or whole tree collapse
Supporting strength	Decline and unstable	Decline and unstable