Black Knot Disease: Symptoms, Treatment, and Prevention

Black knot (Apiosporina morbosa) is a disease that affects species from the Prunus genus of trees and shrubs. This disease creates large black galls and cracks on trees, causing significant economic damage to stone fruit crops, ornamental plants, and the timber industry. The wounds also create opportunities for other diseases to enter the plant and cause secondary infections. 

This wood decay fungi is indigenous to North America and common throughout Canada, the U.S., and Mexico. There has been one reported case of black knot in Taiwan in the 1970s and this disease is considered an EPPO A1 quarantine pest internationally. 

Like other fungal diseases, this disease is hard to eradicate once established. Monitoring trees and frequently pruning diseased branches are key strategies to manage black knot in plum and cherry trees. 

What Is Black Knot Fungus?

Black knot, the common name of this fungus, is the symptom of the disease. The black knot causing fungus Apiosporina morbosa is in the Venturiaceae family, closely related to the fungus that causes apple scab. 

Black knot appears on the woody parts of trees. It is one of the most visible stone fruit diseases as infected trees exhibit large black knot galls that ooze sticky liquid and completely encircle infected branches. Severely infected trees are removed to limit the spread of fungal spores to nearby healthy trees.

Life Cycle

The fungus, Apiosporina morbosa (formerly Dibotryon morbosum), overwinters on the surface of host plants as fruiting structures. These structures on black knot galls release spores in early spring, triggered by wet conditions and mild temperatures.

Transported by wind or rain, black knot spores infect young green shoots and twigs. Wounded branches are also prone to new infections. Similar to other fungal diseases like brown rot, Apiosporina morbosa also instructs tissue to produce extra cells which disrupt normal growth in infected branches. 

The fungus forms fruiting structures on older black knot galls. These continue to produce and release spores every year to prolong the disease cycle. Like other systemic diseases, this one can also travel throughout the plant, causing new fungal growth some distance away from the initial site of infection. 

It is one of the most common plum tree diseases, and also one of the most common cherry tree diseases, among other species.

Symptoms

The telltale sign of galls appears late in the growing season, many months or even years after initial infection. Often, the black knot galls become visible after leaves drop in the fall.

Infected branches may show some initial swelling that can easily go unnoticed. After about a year of infection, the swelling takes on a velvety olive-green covering, transitioning into light brown swellings. The older knots enlarge over time as the fungus grows inside the plant tissue.

Galls vary in size from under an inch to over a foot long. Over time, healthy wood transforms into large hard black knots, which might cause leaves and branches to die. Open wounds and cracks invite new infections.

What Plants Does Black Knot Affect?

Many Prunus trees are susceptible to the disease.

Black cherry (Prunus serotina) is a prized timber species native to North America. Damage and wood rot caused by insects, wounds, and diseases significantly impact the market value of this hardwood. This makes the fungus a particularly troubling disease for the timber industry.

Many ornamental and fruiting cherry and plum trees are susceptible as well, including:

• American plum (P. americana)
• Chokecherry (P. virginiana)
• European “Stanley” plum (P. domestica)
• Japanese plum (P. salicina)
• Sweet cherry (P. avium)
• Mahaleb cherry (P. mahaleb)
• Wild and cultivated species of chokecherry

Control

Because old knots are visible on trees, removing infected twigs or branches is a key control strategy. Carry out this process in the winter when trees are dormant. The lack of leaves makes black knot galls easy to spot.

Cuts should be made four to six inches from woody galls on twigs. When working on larger branches, leave at least one inch of healthy tissue from where you have removed the existing galls.

Use clean pruning tools and best practices to promote healthy regrowth over the pruning wound. Sterilize tools using a solution of at least 70% rubbing alcohol. You may need to consult an arborist to carry out more extensive removals. 

If left uncontrolled, the knots will expand to cover the entire tree. In these cases, the whole tree might need to be cut down. This might be a multi-season project as any visible black knot indicates an infection at least a year old. Burn any clippings and cuttings from infected trees.

Prevention

Copper fungicides can protect trees and young plants from late fall into late winter (roughly Thanksgiving, Christmas, and Valentine’s Day treatment times). A preventative spray of lime sulfur can also be applied when the trees are in dormancy. Do not use these sprays after flowers bloom or before the rain. 

Some Prunus species are less susceptible to black knot disease. For cherry trees, sour cherry (P. cerasus) and Nanking cherry (P. tomentosa) are a few examples. Flowering almond (P. triloba) and apricot (P. armeniaca) are also rarely affected. There are even black knot-resistant plum and cherry trees, such as flowering cherry and president plum.

Wild plum, native plum, and cherry trees can host the black knot fungus and act as a reservoir where the fungus overwinters. Consider removing these wild plants or keep a close eye on them to mitigate spread. 

Frequently Asked Questions