Black Knot Fungus
The disease, Dibotryon morbosom, commonly known as Black knot fungus, gets its name from the black and knotty outgrowth it produces. Infection from this fungus occurs in the spring when spores are transmitted from wind or splashed in the rain onto new growth. The fungus disrupts the normal flow of water and nutrients in mainly plum and cherry trees (as well as the rose family) when growing on the branches and stems, causing dieback within the canopy. Black knot is an extremely difficult fungus to detect during the first year of infection as the symptoms are difficult to see. The infection starts as small swelling of the twig or branch tissue (see picture left). This progresses into the larger black “knots” that represent the disease name.
Spray treatments in spring, done annually for at least 3-5 years, accompanied with pruning and health treatments are the only effective way to combat Black Knot. Black Knot is not an easy disease to control once it has a foothold on the tree. We advise removing any trees that have a canopy with over 30% infection.
As with any disease like this, we highly recommend health treatments regardless of whether your cherry or plum are currently infected. Healthy trees can naturally defend themselves better and can recover from the infection and heavy pruning much easier.