Black knot disease is a common fungal disease affecting trees in the Prunus genus, including many ornamental, edible, and native plum and cherry trees. It is characterized by hard, swollen, black galls (tumor-like growths) that form on branches and, less frequently, on the trunks of infected trees.
Characteristics of Black Knot
- Cause: The disease is caused by the fungus Apiosporina morbosa.
- Symptoms: Easily recognizable by the distinctive black, knot-like growths on twigs and branches. These galls start as small, greenish or brownish swellings and gradually enlarge and darken over time.
- Affected Trees: Common hosts include plum, cherry, apricot, and other Prunus species. Both cultivated and wild varieties are susceptible.
- Progression: The fungal infection typically spreads slowly. However, if left unmanaged, it can weaken the tree, making it susceptible to other diseases and pests. In severe cases, it can lead to branch dieback or even tree death.
- Impact: While many Prunus trees can tolerate black knot, significant infestations can impact fruit production and overall tree health.
Management and Control
- Pruning: The most effective method for controlling black knot is to prune infected branches during the dormant season (late winter or early spring). Cut several inches below the visible gall. Sterilize pruning tools between cuts to prevent spreading the fungus.
- Removal: Severely infected trees may need to be removed entirely to prevent further spread of the disease.
- Fungicides: Fungicides can be used as a preventative measure, particularly during bud break and early shoot growth. Follow the manufacturer's instructions carefully.
- Prevention: Plant disease-resistant varieties of Prunus trees whenever possible. Ensure good air circulation around trees by proper spacing and pruning.
In summary, black knot is a fungal disease that manifests as black galls on Prunus trees and can be managed through diligent pruning and, in some cases, fungicide application.
