Black Knot

What is Black Knot?

Black Knot is a fungal disease predominantly affecting trees from the Prunus genus, including plums, cherries or apricots, and other stone fruits. It’s known for its distinct black, swollen galls or knots that appear on branches and stems.

What Causes Black Knot?

The disease is caused by the fungus Apiosporina morbosa. It overwinters in the knots and releases spores during wet spring weather, which are then spread by wind or rain to new growth.

What are the Symptoms?

The symptoms of this disease can be quite distinctive, making it relatively easy to identify. Here’s how Black Knot manifests on different parts of the tree:

Leaf

• While Black Knot primarily affects woody tissues, nearby leaves can exhibit some indirect symptoms.
• Leaves adjacent to a knot may yellow and wilt, especially if the knot constricts the branch’s vascular tissue.
• In severe infections, premature leaf drop might occur.

Branch

• The most notable symptom appears on branches in the form of elongated, rough, black galls or knots. These can range from a few inches to over a foot in length.
• Initially, these growths are greenish-brown and soft (often referred to as “olive-green swellings”). As they mature, they become hard, brittle, and turn black.
• These galls can completely encircle a branch, leading to wilting and dieback beyond the point of infection.
• Over time, as the fungus grows, these blackened galls become more pronounced, often taking on a gnarly, twisted appearance.

Trunk

• While less common than branch infections, the trunk can also be affected, especially on younger trees or in severe infestations.
• Infected areas on the trunk will also develop the characteristic rough, black galls.
• Large galls on the main trunk can be especially detrimental, weakening the tree and making it more susceptible to secondary infections and environmental stresses.

Whenever you identify these symptoms, especially the distinctive galls on branches, it’s essential to act promptly. Left untreated, Black Knot can severely weaken a tree and make it more susceptible to other diseases and pests.

Black Knot Disease Cycle

The disease cycle of Black Knot is somewhat complex but can be broadly summarized as follows:

• Primary Infection: The disease cycle begins in the spring when ascospores (sexual spores) are released from mature galls during wet weather. These ascospores are spread by wind and rain to susceptible new growth on the host tree.
• Symptom Development: Once the spores land on susceptible tissue, they germinate and infect the young, green twigs. The initial symptoms might be subtle, with small, olive-green swellings appearing on the infected areas.
• Gall Formation: As the season progresses, these swellings become more prominent and turn into dark, black, elongated galls by the end of the first year.
• Overwintering: The fungus overwinters within these galls.
• Spore Production: In the second year, during wet periods in the spring, the galls produce more ascospores which are then ready to infect new growth.
• Secondary Infection: If the disease is left unchecked, older galls can produce spores for several years, acting as a source of infection for surrounding trees.
• Disease Progression: As the galls grow, they can girdle branches, leading to dieback beyond the point of infection. Over time, heavily infected trees can become severely weakened.

To manage Black Knot, it’s crucial to understand this disease cycle, as interventions, like pruning infected branches, are most effective when timed to disrupt the cycle.

Regions Impacted

Black Knot is predominantly found in North America, particularly in the eastern and midwestern U.S. and eastern Canada. The disease thrives in regions with wet spring weather, as moisture favors fungal growth and spore dissemination.

Damage and Detection

• The most evident sign of Black Knot is the presence of the distinctive black galls on branches.
• Severely infected trees might experience dieback, stunted growth, or even death.
• While young trees can be severely affected or killed, older trees, though disfigured by the disease, usually survive.

How to Control and Prevent Black Knot?

Controlling and preventing Black Knot is vital for maintaining the health and aesthetics of susceptible trees. Here’s a comprehensive strategy to tackle Black Knot:

• Regular Inspection: Begin with routine inspections of trees during the dormant season, looking for characteristic dark, elongated galls.
• Pruning: Once detected, prune out infected branches, cutting at least 4-6 inches (10-15 cm) below the visible gall to ensure all infected tissue is removed. Use clean, sharp pruning tools and sanitize them between cuts with a 10% bleach solution or rubbing alcohol to prevent the spreading of the disease.
• Proper Disposal: Do not compost the pruned material. Burn, bury, or bag and dispose of the galls to prevent them from releasing more spores.
• Fungicides: While pruning is the primary control method, fungicides can offer additional protection, especially during wet springs. Products containing chlorothalonil or lime sulfur can be effective. Begin applications when buds start to break and continue at intervals specified on the label until the end of the wet season.
• Tree Health: Promote the overall health of the trees. A healthy tree is better equipped to resist infections. Ensure proper watering, mulching, and avoid injuring the tree.
• Resistant Varieties: If planting new trees, consider selecting resistant varieties or species. While no cherry or plum tree is entirely immune, some cultivars show a higher resistance to Black Knot than others.
• Neighborhood Watch: If Black Knot is prevalent in your neighborhood or nearby areas, collaborate with neighbors to control it. An infected tree nearby can be a source of infection for your trees.
• Clean Up Fallen Debris: Regularly clean up and dispose of fallen leaves and fruit from the tree’s base. While these don’t spread Black Knot, general sanitation can prevent other diseases and pests.

Remember, while these measures can significantly reduce the incidence of Black Knot, it’s challenging to ensure complete protection, especially if the disease is prevalent in your region. Regular monitoring and a combination of the strategies above will offer the best results.