The Silent Killer: Protecting Trees from Black Knot Fungus

At first glance, it might just look like an odd lump or a harmless dark growth on tree branches. But Black Knot Fungus is a silent killer, slowly strangling trees from the inside out. This sneaky fungal disease is notorious for infecting cherry and plum trees, creating grotesque, coal-like swellings that weaken branches and, if left untreated, can lead to the tree’s demise. The worst part? By the time you notice the damage, it’s often already spread.

In this guide, we’ll discuss how to identify, manage, and prevent Black Knot to protect the health of our city trees.

What Is Black Knot Fungus

Black Knot Fungus (Apiosporina morbosa) is a fungal disease that primarily affects Prunus species, making it a concern for urban forestry.

The tree disease develops slowly, often going unnoticed for years until dark, swollen growths appear along branches. These hard, black, irregular knots gradually expand, restricting nutrient flow and weakening the tree.

As the infection spreads, it deforms branches, causing dieback and structural instability. If left unmanaged, the fungal infection can compromise entire tree populations, making early detection and intervention essential.

Symptoms to Watch Out For

Some symptoms include:

• Early-stage swelling: Small, greenish-brown bumps form on young branches, often mistaken for normal growth.
• Mature black knots: Over time, the swelling hardens into thick, black, tar-like masses, wrapping around branches.
• Cracked or peeling bark: Infected branches may develop cracks in the bark near the knots, sometimes peeling away to reveal dark, decayed wood underneath.
• Deformed branches: Infected limbs become twisted, swollen, and irregularly shaped, affecting tree structure.
• Brittle and weakened wood: As the knots expand, branches become fragile and prone to snapping, especially in strong winds.
• Widespread dieback: Severe infections restrict nutrient flow, leading to dead branches and canopy thinning.

What Causes Black Knot and How Does It Spread?

This fungal disease spreads primarily through airborne spores, which are released in spring and early summer when moisture levels are high.

These spores land on new growth, infecting young branches before visible symptoms appear. The infection progresses slowly, making early intervention challenging in dense urban landscapes.

Several factors contribute to the rapid spread of the Black Knot in municipalities:

• Overcrowded Prunus species: Trees planted too close together allow spores to transfer easily from one tree to another, increasing the risk of outbreaks.
• Lack of routine pruning: Infected branches that remain on trees act as unending sources of spores, spreading the disease to healthy areas.
• Prolonged wet conditions: Mild temperatures combined with humidity create the perfect environment for spore germination, accelerating infection rates.
• Improper disposal of infected branches: If removed limbs are left on-site or chipped into mulch, spores can continue to spread. Proper disposal methods, such as burning or deep burial, are essential.
• Use of infected nursery stock: Planting trees that already carry latent infections introduces Black Knot into new areas before symptoms appear.
• Tree wounds or stress: Pruning wounds, storm damage, or weak growth can create entry points for spores, making stressed trees more vulnerable.
• Lack of tree diversity: Urban landscapes that over-rely on Prunus species create a perfect environment for Black Knot outbreaks, as the fungus can spread easily between genetically similar trees.
• Slow municipal response: Without timely tree inspections and removals, Black Knot can establish itself across parks, streets, and residential areas, requiring more extensive interventions.

What Can Be Done to Control The Spread of Black Knot

1. Pruning of Infected Trees

One of the most effective ways to control Black Knot is with strategic pruning. Infected branches must be removed at least 6–8 inches below the visible knot, as the fungus extends beyond what’s visible on the surface. Cutting too close to the infected area can leave behind fungal spores, allowing the disease to persist.

Timing is critical. Pruning should be done in late winter, when the fungus is dormant, to minimise the risk of spore release. If pruning occurs during the growing season, spores can easily spread to nearby trees, worsening the problem.

2. Disposal of Branches

Proper disposal of infected branches is just as important as removal. Simply cutting and leaving diseased limbs on-site can lead to reinfection.

Best disposal practices include burning, deep burial, or completely removing infected wood from the area to prevent further contamination. We don’t recommend chipping infected wood, unless the mulch is used far from Prunus species.

3. Strategic Tree Planting and Selection

Municipal tree programs can reduce Black Knot outbreaks by carefully selecting and placing tree species. Since Prunus trees are highly susceptible, they should be planted sparingly in high-risk areas where Black Knot is already prevalent. If an area has a history of Black Knot infections, it’s best to choose alternative tree species with higher resistance.

Diversity is also a key factor in urban forest resilience. Planting a wide variety of species instead of over-relying on Prunus trees prevents disease from spreading across entire neighbourhoods. Cities should prioritise mixed plantings in parks, streetscapes, and public green spaces to reduce vulnerability.

4. Fungicide Applications (Preventative, Not Curative)

Pruning is the go-to method to control Black Knot, but fungicide treatments may be useful for valuable plantings, such as collections, orchards, arboreta, or in the event of severe infestations.

Fungicides should be applied before bud break in early spring and immediately after pruning to protect fresh cuts from infection. However, fungicides cannot cure Black Knot; they are only effective in preventing new infections.

Because of the challenges associated with large-scale spraying, municipalities should prioritise fungicide applications for newly planted or high-value trees rather than relying on them as a sole management strategy.

5. Public Awareness and Municipal Monitoring

Black Knot control requires community involvement. Arborists should conduct regular tree inspections to identify early signs of infection before the disease becomes widespread. Early detection allows for timely pruning and containment efforts, reducing long-term costs.

Public education is also a powerful tool here. Raising awareness about identifying Black Knot and the best ways to deal with it can help the community.

How Green Drop Helps Municipalities

Managing disease on a municipal scale requires expertise, strategic planning, and consistent action. Our ISA-certified arborists specialise in urban tree health, offering municipalities proactive solutions to control and prevent outbreaks before they compromise city trees.

We provide:

• Comprehensive tree health care solutions to keep trees strong and thriving through the year.
• Tree assessments and disease diagnostics, identifying infections early to prevent widespread damage.
• Tree planning and planting recommendations, helping cities build more resilient urban forests.
• Precise pruning and removal of infected branches, ensuring the fungus doesn’t spread while following best disposal practices.
• Tree removal services when a tree is too far gone to be recovered.
• Ongoing municipal tree monitoring and maintenance programs, reducing future disease outbreaks through proactive care.

Don’t Let Black Knot Take Over—Act Now!

Black Knot is a persistent threat to urban forests, but early intervention and strategic management can prevent widespread damage.

Our ISA-certified arborists provide expert tree care solutions for municipalities across Edmonton, Calgary, Red Deer, Saskatoon, Winnipeg, and Regina.

Whether your city needs disease management, tree assessments, or pruning services, we’re here to help. Let’s work together to protect urban trees – contact us today to develop a customized tree health strategy for your community.