Besides being unsightly, black knot considerably reduces fruit production. But fortunately, the disease develops slowly over the course of several years, and if caught early, it can be controlled by monitoring and pruning.
What Is Black Knot?
Black knot is a fungal disease caused by the fungus Apiosporina morbosa (formerly referred to as Dibotryon morbosum). It affects mainly plum and cherry trees in North America.
The disease has been reported on about two dozen Prunus species. The following species and their cultivars can get black knot: American plum (Prunus americana), European plum (Prunus domestica), Japanese plum (Prunus salicina), sweet cherry (Prunus avium) and Mahaleb cherry (Prunus mahaleb), as well as wild and cultivated species of chokecherries (Prunus virginiana).
During rainy weather between the months of April and June, a tree infected with the fungus ejects spores that transmit the fungus to new, susceptible tissue on the same tree, or to other trees.
Spring temperatures between 60 to 80 degrees F are ideal for spreading the disease. The infective spores are carried by the wind or by rain splashes.
At the infected locations, the tree won’t grow normally but will form tumor-like tissue instead. During the first year, these are barely noticeable swellings or small galls.
The fungus then overwinters on the tree. During the spring and summer of the second year, the infected tissue grows rapidly and forms knots around the stem or branch. Earlier in the season, the knots are greenish-brown in color and have a cork-like appearance, then they gradually darken.
In its second winter, the fungus again develops infective spores that will attack new healthy tissue. The knots and the bark split, allowing the spores to be released.
If the disease is untreated and the infected branches are left on the tree, the black knots continue to grow and produce more infective spores in the following years. The production of new infective spores will continue as long as the tissue is alive.
Signs and Symptoms
During the first year after the tree has been infected, black knot is not obvious because the small, light-brown swellings do not stand out, and the infected parts are often covered by dense foliage. The next spring, however, when the knots grow rapidly and darken to their typical coal-black color, the disease is more easily identified.
The knot size ranges between half an inch to 12 inches in length and up to two inches in circumference. The tips of infected twigs are often lop-sided, and a sticky liquid may ooze out of the cracks in the knots and bark.
Infected branches may wilt, won’t leaf out in the spring, or die. A heavily infected tree has numerous knots.
Treating and Managing Black Knot
If you are growing any of the prunus species that are possible hosts for the black knot fungus, monitoring your trees is crucial. Inspecting the trees during dormant season is when you can see best what’s going on. Pruning is your first line of defense against black knot, and it should be done during the winter before the disease cycle continues.
If you spot any galls, remove the infected branches at least six inches, if possible up to 12 inches away from the knot. Generously pruning out the infected parts ensures that you also remove the vegetative part of the fungus (mycelium) that does not visually appear as infected yet but will continue to grow if left on the tree.
Even after the knots have been removed from the tree, they can produce infective spores so make sure to safely discard the infected stems and branches by throwing them in the trash or burning them.
Also, sanitize your pruning tools with 70 percent rubbing alcohol to prevent reinfection. If a branch has numerous large black knots, remove as much or all of the branch until you reach healthy tissue. A severely infected tree is best to be cut down completely.
After pruning, continue to monitor the tree in the following months and years to detect newly infected parts early. It is also a good idea to check if any wild Prunus host trees in nearby locations, such woodlands, are infected.
Preventing Black Knot
If you live an area where wild Prunus species are frequently infected with black knot, planting species that are prone to get the disease is not recommended.
Before planting a tree, inspect the surroundings for the disease both in existing landscaping and in nearby empty lots and woodlands.
Another way of preventing black knot is to select cultivars with black knot tolerance or resistance, such as Damson or Santa Rosa plums. When purchasing a new tree, inspect it for signs of the disease.
If monitoring and controlling black knot through monitoring and pruning fails, or to protect young trees that might likely be infected, fungicide sprays (captan, chlorothalonil, thiophanate-methyl, lime sulfur) may be applied.
These fungicides should be applied in early spring before bud break and at regular intervals until the new shoots have matured. Make sure that the label specifies the use of the fungicide for your particular Prunus variety, and that it is registered for use on edible Prunus cultivars if that’s what you are using it for.