The following information on Black Knot has been included to assist you with identifying and understanding the best methods to control Black Knot on your trees.

We take measures to control Black Knot when observed on any of our City trees in park areas and boulevards as it spreads rapidly to surrounding trees of the Prunus variety.

Black Knot: Caused by a pathogen known as Apiosporina Morbosa. It is an air born fungus, which can be very contagious. Spores move to other trees by wind and birds.

Black Knot Affects: At least two dozen species of cherries, plums and other members of the genus Prunus. It is common on wild chokecherry, plums, flowering almond, apricot, blackthorn, cherries such as nanking, pin, sand, bird, bitter, black, mahaleb, sour and sweet, peach plums such as American, beach, Canada, common, damson, Japanese, myrobalan and sierra.

Black Knot Symptoms: Cankerous swellings on infected areas of current season’s growth that infect the previous spring. Winter dormancy interrupts the cankerous growth and in spring, the aeciospores eject into the air in response to rain.

Black Knot Damage To Infected Tree: Unsightly galls or cankerous knots, which will girdle branches and cause them to die. The duration of the disease cycle is normally two years.

Black Knot Control On Your Trees and Shrubs: Pruning knots and galls from tree by cutting 10cm beyond the swelling. After pruning, be sure to disinfect pruning tools with a mixture of bleach and water. Burn diseased material immediately or put branches in a sealed garbage bag and take to the landfill.


Tree with Black Knot

Photo credit: Tricia Simon via

Black Knot closeup

Photo credit: Tricia Simon via

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