No, Apiosporina morbosa (commonly known as black knot fungus) is not harmful to humans. You do not need to worry about any health risks from touching the galls or inhaling its spores.
What is Black Knot Fungus?
Black knot fungus is a common and often destructive fungal disease that primarily affects trees and shrubs in the Prunus genus, which includes various stone fruit trees such as:
- Cherries (e.g., sour cherry, sweet cherry, ornamental cherry)
- Plums (e.g., wild plum, cultivated plum)
- Apricots
- Peaches
The fungus creates distinctive, hard, black, warty growths or "knots" on branches, twigs, and sometimes even the main trunk of infected plants. These galls can range in size from small swellings to elongated masses several inches long.
Impact on Plants
While harmless to humans, black knot fungus can severely damage and even kill susceptible trees if left untreated. Its primary impact on plants includes:
- Reduced Vigor: The galls disrupt the flow of water and nutrients within the affected branches, weakening the tree.
- Branch Dieback: Infected branches often die back beyond the knot, leading to a decline in tree health and productivity.
- Reduced Fruit Production: In fruit-bearing trees, the disease can significantly reduce the quantity and quality of the fruit.
- Tree Mortality: Severe infections can girdle limbs or the main trunk, eventually leading to the death of the entire tree.
Managing Black Knot in Your Garden
Although black knot fungus poses no threat to human health, proper management is crucial for the well-being of your Prunus trees.
Here are practical steps for dealing with infected plants:
- Pruning: The most effective control method is to prune out infected branches during the dormant season (late winter to early spring) before new growth begins. Make cuts at least 6-8 inches below the visible knot into healthy wood.
- Tool Sanitation: Always disinfect pruning tools with a 10% bleach solution or rubbing alcohol between cuts and between trees to prevent the spread of spores.
- Proper Disposal: Dispose of all pruned knot material immediately. Do not leave it on the ground, and do not compost it, as spores can overwinter and reinfect trees. Burning or burying are effective disposal methods.
- Fungicides: In some cases, fungicides can be used as a preventative measure, especially for highly susceptible varieties or in areas with high disease pressure. Consult with a local arborist or extension service for recommended products and application timings.
- Resistant Varieties: When planting new trees, consider choosing varieties that are known to have some resistance to black knot.
For more detailed information on identifying and managing black knot fungus, you can consult reputable horticultural resources such as university extension services. An example of such a resource can be found via the University of Minnesota Extension on Black Knot.
