Black Knot is a widespread disease caused by the fungus Apiosporina morbosa. This pathogen creates distinct, unsightly growths on the branches of many popular backyard trees, especially fruit-bearing trees like plums and cherries. If left unmanaged, the fungus is a serious threat to the health and aesthetics of affected woody plants, often leading to branch death. This article addresses the nature of Black Knot and whether this common plant pathogen poses any danger to human health.
What Black Knot Fungus Looks Like and Who It Targets
Black Knot is easily identified by the characteristic hard, black, cylindrical swellings that form along the twigs and branches of susceptible trees. These growths begin as small, olive-green swellings in the spring, often going unnoticed for a year or more after the initial infection takes hold. Over time, these growths harden into a rough, tar-like texture and can range in size from half an inch to over a foot long, sometimes completely encircling a branch.
The fungus is a highly host-specific plant pathogen, meaning it only targets a narrow range of organisms for infection. Black Knot exclusively affects trees and shrubs belonging to the Prunus genus, including species like plums, cherries, apricots, and almonds. Its life cycle involves releasing spores during wet spring weather, which are then carried by wind and rain to infect young growth or wounds on nearby trees. This fungus is biologically adapted to infect the tissues of woody plants, not the cells of humans or other mammals.
Direct Risk Assessment for Human Health
Black Knot fungus is not considered toxic, poisonous, or infectious to people or domestic pets. Biologically, the fungus Apiosporina morbosa is classified as a plant pathogen and lacks the necessary mechanisms to cause systemic infection in mammals. There are no documented cases of Black Knot causing disease, poisoning, or allergic reactions in humans.
Concerns about ingestion are common, particularly when children or pets encounter infected branches. While ingesting any non-food item carries a minor risk of gastrointestinal upset, the fungus itself is not known to be acutely poisonous. Most health organizations consider the risk of Black Knot ingestion to be negligible to low for both humans and animals.
Inhalation of the fungal spores, which are released from the galls during wet periods, is not known to cause respiratory illness. The spores are primarily a mechanism for plant-to-plant spread and do not function as human respiratory allergens or infectious agents. Direct contact with the hard, black galls on the tree is generally safe, as the fungus does not possess caustic or irritating properties toward human skin. Standard gardening precautions, like wearing gloves, are recommended when handling diseased or rough plant material.
Safe Handling and Disposal Procedures
Although Black Knot is not a health risk to people, managing it is important for the longevity of the affected tree. The removal process should be performed during the dormant season, typically in late fall or winter, when the fungus is less actively producing new spores. When pruning an infected branch, make the cut at least six to twelve inches below the visible gall to ensure the entire fungal growth within the wood is removed.
Sanitizing pruning tools is a necessary step to prevent the accidental spread of the fungus to healthy branches or other susceptible trees. Tools should be wiped down with a solution of diluted bleach or isopropyl alcohol between each cut. This simple action prevents the transfer of microscopic fungal material that could initiate a new infection site.
Proper disposal of the removed galls is the final step in controlling the disease. The infected wood must be destroyed, as the galls can continue to produce and release viable spores for up to four months after being cut from the tree. The safest methods for disposal are burning the material, where local ordinances allow, or sealing the infected pieces in a trash bag for landfill disposal. Composting the material is not recommended because the fungus can survive and spread spores from the compost pile.