Black Knot Fungus: Why Dormant-Season Pruning Matters

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Black Knot Fungus on Prunus species

black knot fungus on prunus species

Black knot fungus is a common and serious disease affecting cherry and plum trees (Prunus species) throughout British Columbia. Caused by Apiosporina morbosa, it appears as hard, black, swollen growths on branches that restrict sap flow, weaken structure, and lead to progressive dieback if left untreated.

In our region, cool and wet spring conditions create ideal circumstances for the disease to spread—making early intervention and correct timing essential.

Pruning Is the Most Effective Control

Arborist Pruning Black Knot fungus

pruning black knot fungus

There is no cure once black knot is established. Pruning out infected wood is the most reliable way to reduce spread and protect the tree.

Effective pruning:

  • Removes active sources of spores

  • Improves airflow and overall tree health

  • Reduces long-term structural damage

However, pruning at the wrong time can increase the risk of reinfection.

Dormant Season Timing Is Critical

The best time to prune black knot is during dormancy, typically from late fall through winter, before bud break.

Dormant pruning:

  • Minimizes spore spread

  • Allows clear visibility of infections

  • Reduces stress on the tree

Pruning during spring or summer—especially in wet conditions—can unintentionally spread the disease through fresh cuts.

Proper Technique Matters

Disinfect Tools

Disinfect Tools

Black knot must be removed 15–20 cm (6–8 inches) below the visible infection into healthy wood. Tools should be disinfected between cuts, and infected material must be removed from site—not composted.

Incomplete removal or missed infections allow the fungus to return quickly.

Act Before the Spring Rush

Dormant season is the time to deal with fungal issues on trees to avoid further spread.

Now is the ideal time to schedule a tree assessment with our ISA Certified Arborists, while trees are still dormant and before the spring rush begins. An early assessment allows for proper timing, safer pruning, and better long-term results for your trees.

If you suspect black knot or want peace of mind before spring, book your assessment now and get ahead of the season. 🌲

Schedule Tree Health Assessment