What is black knot fungus?

Black knot fungus is a disease that affects cherry, plum, apricot trees and other varieties from the prunus family. The disease appears as black, swollen knots on branches and twigs of affected trees. These knots are typically several inches long and can cause the branch to girdle. When left untreated the fungus will strangle new growth. This can cause the branch to weaken or die due to a lack of water and nutrients being able to flow freely. Additionally, the disease provides gateways for other disease and pest infestations.

How does it spread?

Black knot fungus spreads when spores are released in wet periods. Insects, wind, and unsterilized tools can spread the disease during this time.

How to spot it

Black knot fungus develops slowly, so unfortunately it will typically have the chance to grow for a season before being visibly noticeable. For about the first year after infection begins, it is still difficult to spot due to the swellings being small and brown or green in color. If you miss those initial signs, then it will be much easier to spot. After a couple years of developing, the swellings are hard, black, and much bigger in size, about 10-15 cm.

How to treat it

• Prune infected branches. Be sure to destroy the removed branches to prevent spread.
• Sterilize pruning tools. You must sterilize the pruning tools between each cut to prevent the spread of the disease.
• Use a fungicide. Choose a fungicide product that is specifically formulated to target black knot.  Follow the instructions provided on the packaging. Wear appropriate protective clothing to apply the fungicide. Take extra care during application to avoid sensitive areas such as waterways, other plants, or areas where pets or children may play.
• Keep your trees watered adequately. Inadequate watering or over fertilizing can increase the risk of black knot infection.

If you’re unsure whether or not you have a black knot fungus infestation, contact us for a free assessment and quote.