The Role of Fungi in Tree Health: Identifying the Dangers and Managing the Threats
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The Role of Fungi in Tree Health: Identifying the Dangers and Managing the Threats

Trees and fungi have a complex relationship that spans millions of years, with most interactions being mutually beneficial. Fungi play a vital role in nutrient cycling, helping trees absorb nutrients from the soil through symbiotic relationships like mycorrhizae. However, not all fungi are friendly. Some, particularly wood-decay fungi, can attack living trees, causing structural instability, which may pose risks to people and property.

This article explores the role of fungi in trees, with a focus on identifying the more dangerous ones, including Armillaria mellea and Kretzschmaria deusta. We’ll look at their effects on tree health, the potential consequences, and the appropriate management responses for arborists and tree owners alike.

Fungi: A General Overview

Fungi are neither plants nor animals; they belong to a separate kingdom and play diverse roles in nature. For trees, fungi can be both beneficial and detrimental:

  • Symbiotic fungi: Some fungi form partnerships with trees, aiding in nutrient absorption and promoting tree health. For example, mycorrhizal fungi exchange nutrients with trees, improving access to minerals like phosphorus and nitrogen.
  • Saprophytic fungi: These fungi feed on dead organic matter and help decompose fallen wood. They generally don’t harm healthy trees.
  • Parasitic fungi: The dangerous group, these fungi attack living trees, causing diseases or decaying structural wood. Left unchecked, they can lead to significant damage, both to the tree and nearby infrastructure.

When Fungi Turn Dangerous

While many fungi are harmless or even beneficial, parasitic fungi can attack living trees. These fungi infect the roots, trunk, and branches, causing decay, structural instability, and ultimately, tree failure. In many cases, the damage can go unnoticed until it becomes advanced, leading to hazardous situations.

Key Fungi of Concern

1. Armillaria mellea (Honey Fungus)

  • Common Name: Honey Fungus
  • Description: Recognizable by its honey-colored caps and black bootlace-like rhizomorphs, Armillaria is one of the most infamous tree-killing fungi. The fruiting bodies typically appear at the base of infected trees during late summer and autumn.
  • Trees Affected: It can affect a wide variety of trees, including oaks, elms, fruit trees, and birch.
  • Effects: Armillaria attacks the root system and cambium layer (the active, growing part of the tree), causing the tree to lose its structural integrity. The infection often leads to rapid tree decline and death.
  • Consequences: The decay caused by Armillaria can lead to tree failure, making infected trees a potential hazard. If the infection spreads to nearby trees, it can create significant ecological and financial costs.

2. Kretzschmaria deusta (Brittle Cinder Fungus)

  • Common Name: Brittle Cinder Fungus
  • Description: This fungus appears as flat, greyish-black crusts on the lower trunk, resembling burnt charcoal. Internally, it causes a type of soft rot that can be difficult to detect in the early stages. When the fruiting bodies are visible, significant decay has usually occurred.
  • Trees Affected: Commonly found on beech, lime, sycamore, and horse chestnut.
  • Effects: Kretzschmaria deusta weakens the tree’s base by causing soft rot, especially around the root collar and lower trunk, leading to structural instability. The tree may collapse without warning due to hidden internal decay.
  • Consequences: Brittle cinder fungus is particularly dangerous because the visible signs are often delayed, and the tree may fail without much external evidence of decay.

3. Ganoderma spp. (Artist’s Bracket)

  • Common Name: Artist’s Bracket Fungus
  • Description: This bracket fungus appears as a large, shelf-like growth with a brown upper surface and white underside. The underside can be etched or "drawn" on, hence the common name.
  • Trees Affected: Primarily affects oaks, maples, and elms.
  • Effects: Ganoderma causes heart rot by breaking down lignin in the wood, leaving the tree vulnerable to breakage.
  • Consequences: While trees can survive with Ganoderma for years, advanced decay weakens the tree from the inside, making it prone to collapse under high winds or other stress factors.

Detecting Fungal Infections and Managing Risk

One of the challenges with parasitic fungi is that the decay often remains hidden until it’s too late. As part of a comprehensive tree care program, regular inspections by a qualified arborist are essential.

Early Detection and Monitoring

  • Visual inspection: Regular monitoring of trees, particularly the root collar and lower trunk, for fruiting bodies or other signs of decay.
  • Tree surveys: Professional tree surveys using tools such as resistographs or sonic tomography can reveal the extent of internal decay.

Reactive Works: When and What to Do

Depending on the level of infection and decay, different management strategies may be necessary:

  1. Monitor and Manage: If the infection is in its early stages and the tree remains structurally sound, regular monitoring may be sufficient. Trees can coexist with fungi for years without posing immediate risks.
  2. Pruning: In cases where the infection is localized (e.g., in a single branch), targeted pruning can remove the infected area and limit further spread.
  3. Felling and Removal: In cases of advanced infection or when the tree poses a significant risk to safety (e.g., Armillaria or Kretzschmaria infections), tree removal may be the only option. Removal is often necessary if there’s a risk of sudden collapse or if the fungus spreads to nearby trees.

Soft Rot and Hidden Dangers

Soft rot, caused by fungi like Kretzschmaria deusta, is particularly dangerous because it develops internally, leaving the tree structurally compromised but appearing relatively healthy externally. Regular inspection, including the use of tools like resistographs, is essential for detecting such hidden decay.

When to Leave the Tree Alone

Not all fungal infections require immediate action. Some trees can live for years with fungal decay without posing any risks. For instance, while Ganoderma causes significant internal decay, trees with these fungi often survive for long periods, especially if they are not in high-risk areas like near buildings or walkways.

Conclusion: A Balanced Perspective on Fungi and Trees

Fungi play a crucial role in the ecosystem, supporting the health of forests and urban landscapes. While most fungi are beneficial or neutral, some parasitic species can compromise the structural integrity of trees, posing risks to safety. Regular tree inspections, early detection of decay, and professional management practices can help mitigate these risks.

By understanding the role of fungi and being proactive in tree care, we can preserve trees while ensuring safety in our shared environments. If you suspect fungal infections in your trees, consult a qualified arborist to assess the situation and recommend appropriate actions based on current guidelines from the Arboricultural Association and other industry bodies.