Can you help me identify if this is Black Knot fungus? Thank you, Andy Richter

Asked May 22, 2017, 9:39 AM EDT

Can you help me identify if this is Black Knot fungus? Thank you, Andy Richter

Butler County Ohio

1 Response

Hi - Thank you for your inquiry. This is back knot. If you would like to talk further about this you can give me a call. In the meantime, here is a nice synopsis from some of my Extension colleagues in Colorado, "Black Knot is a very common, and severe, fungus problem that attacks members of the Prunus family including plums, cherries, apricots and chokecherries. The fungus, Apiosporina morbosa, attacks new growth on branches. Black Knot galls are a combination of plant tissue and fungal tissue growing together, with the outer layer being primarily fungal. Infection is at its peak when young shoots are growing and temperatures are between 70-75°F.

The fungus chokes off the flow of water and nutrients in the tree, causing the tree to become weak and stunted. If left untreated, the tree will become extremely unsightly and eventually the fungus will cause major structural weakness or may kill the tree.

Manage Black Knot infections by pruning out knots and swellings before shoot growth starts in the spring, usually in late February through March. To remove Black Knot galls, cut the branch at least 6 to 8 inches below the swelling into the healthy wood to ensure that all of the fungus is removed from the tree. ALL knots and first-year swellings must be removed.

Using 70% ethanol or isopropanol-rubbing alcohol (bleach will rust pruners), always disinfect pruning shears before and after each cut of this diseased plant material. Pruned out limbs must be removed from the area and destroyed because spores will develop on pruned material. The fungus can travel at least 500 feet, so disposal is important.

Fungicide spray in spring to early summer can also help manage Black Knot. However, spray programs are only useful if knots and swells are removed and you locate the source of the fungus. Your tree may have received the fungus from another infected tree at least 500 feet away, so consider looking for the source.

There is no cure for the disease, and once a tree has the disease, it will always have the disease. Most times, the infections will come back and are difficult to detect in the first year with only a slight swelling on the trees. Some homeowners consider this 'a losing battle', and have their trees removed.

The University of Minnesota has some good information on Black Knot:

Donnetta Wilhelm