Overview:
Black knot is a cankering fungus disease that infects many trees and shrubs within the genus Prunus. It is caused by the native North American fungal pathogen Apiosporina morbosa. Although black knot fungal galls have been found in a couple of dozen Prunus species, not all species are equally susceptible, and it can be most destructive to certain varieties of cherries and plums (Photo 1).
The following species can be highly susceptible to infections: Japanese plums (Prunus salicina), European and American plums (Prunus domestica & Prunus americana), Sweet cherry (Prunus avium), Mahaleb cherry (Prunus mahaleb), and cultivated species of chokecherries.
This blog reviews the symptoms and signs of Black Knot Disease. It’s management by pruning and fungicide treatments is discussed. Finally, a handful of resistant plum varieties is provided.
Photo 1: This Japanese Plum species is highly susceptible to Black Knot Disease. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)
Symptoms:
Individual trees can have just a few galls or several hundred. Trees heavily infected can begin to show significant branch dieback with considerable shoot & leaf death. Some landscape plants can be so severely infected that most of the tree branches are killed, eventually causing death to the tree (Photo 2).
Photo 2: This plum tree was eventually killed by Black Knot after many years of infections. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)
The new infections occur on the soft & succulent terminal twig growth. The 1st year symptoms are virtually imperceptible during the initial growing season. After the first-year branch swellings, the tumor-like black knots begin to grow after a couple of seasons. The normal branch growth becomes disrupted & the distorted knots start to form (Photo 3). Towards the end of the second year, the enlarging galls can begin to split open & develop black knots. These swollen, distorted, black knotty growths become especially apparent during the second winter after infection & leaves have defoliated (Photo 4).
Photo 3: Plum tree branch girdled & killed by Black Knot Disease infection. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)
Photo 4: Black Knot symptoms are most apparent during the winter season after leaves have defoliated. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.).
Signs:
The fungal gall growths that occur during the second year after infection will be on young twigs or along the edges of older galls. These swelling areas are covered with velvety, olive-green, or reddish-brown galls (Photo 5). On older twig growth, the galls can elongate many inches in length over several years. In some cases, galls can eventually reach over a foot in total length along the branches. Death to branches can occur if they become girdled over the years (Photo 6).
Photo 5: Second year Black Knot gall growths will have olive-green or reddish-brown colorations. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)
Photo 6: Black Knot galls after multiple years of new infection growth reaching a foot in length. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)
It is during the 3rd spring season that the galls can begin to release spores. The black knot fungi release spores during wet spring periods. The spores can be spread by rain splash or the wind. Infection to new young green growth or wounded branches can occur during these times (Photo 7).
Photo 7: Three year old Black Knot galls can begin to release spores that can be spread by wind & rain splash. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.).
As the black knot galls continue to enlarge during subsequent years, they can eventually girdle stems & cause their death. If infected branches are not killed, then the galls can continue to enlarge each year & produce new fungal spores (Photo 8).
Photo 8: Black Knot newer gall growth adjacent & left of the old gall knots. If infected branches are not yet killed, then newer galls can continue to enlarge each year. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)
Tree Decline:
Heavily infected tree canopies can eventually contain hundreds of black knot galls. The enlarging corky knots cause branches to bend and grow at odd angles that become highly distorted. Over numerous years, the knots can expand half a foot or more in length (Photo 9). When infections to highly susceptible trees are left unmanaged, then symptoms will worsen each year with the decline and death of multiple branches. Older black knot galls can sometimes show evidence of having wood-boring insects.
Photo 9: Black Knot gall causing twisted & distorted branch (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)
Although less commonly observed, black knot galls can sometimes be seen on main tree trunks (Photo 10). The galls produced on trunks of large trees can sometimes reach the size of basketballs. These infections are more commonly seen with wild black cherry, (Prunus serotina). These large trunk galls can grow & enlarge for decades on some trees (Photo 11). When infections to the trunk or main stem continue to advance, then tree death is more rapid & inevitable. When trees become seriously infected, they should be cut down & removed.
Photo 10: Black Knot gall infection on main tree trunk of wild black cherry. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)
Photo 11: Black Knot on trunk of large wild black cherry tree. The Black Knot infection on this tree has probably been enlarging for over a decade. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)
Although black knot fungus will not infect the heartwood of affected trees, it can cause rough cracks in the trunk. These trunk openings can allow other wood-decaying fungi to invade the tree & cause rot. At times, a colorful, liquid-sticky ooze can emerge (Photo 12 & Photo 13).
Photo 12: Black Knot infections over the years can cause extreme rough cracks in bark tissues. Can provide entrance to other wood-decaying fungi. (Phot Credit: Steven K. Rettke, Rutgers Coop. Ext.)
Photo 13: Black Knot galls causing a colorful, liquid-sticky ooze to emerge from infected area. (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)
It is common to observe a white or pinkish-colored fungus growing on the surface of older blackened galls. These are caused by the mycoparasitic fungus Trichothecium roseum (Photo 14). These fungi do not infect the Prunus tree but are only growing on the black knot fungi.
Photo 14: Black Knot & mycoparasitic fungus (Trichothecium roseum) growing on older galls (Photo Credit: Steven K. Rettke, Rutgers Coop. Ext.)
Management (Pruning):
Black knot disease infections can be managed through a combination of cultural pruning & chemical fungicide options. Trees showing recent signs of developing galls should be monitored in winter to prune out these visible growths.
The removal of the galls will help reduce the inoculum levels as spores are released during wet spring periods. Pruning cuts should be at least 6 inches below the infective knot areas. Disinfecting the pruners between cuts with 70% alcohol or a 10% bleach solution will help prevent pathogen spread.
Since black knot galls can continue to release spores after being pruned out, they need to be removed from the site. Also, removing the black knots from other infected trees on nearby sites will improve suppression.
Management (Fungicides):
Rain splash & the wind can carry spores that infect new green tissue growth. Preventative fungicide sprays can effectively reduce the spread of new growth with trees that are only mildly infected. The application of fungicides during the period of active shoot elongation in the spring can provide further protection but may be of limited value without the cultural procedures mentioned above. Examples of effective fungicide materials include sulfur & copper products, Captan, Chlorothalonil, & Thiophanate-methyl. Treatments should be applied at bud break & then every 7-14 days until new terminal growth ends. Timing sprays before anticipated rains is especially valuable.
Final Thoughts & Resistant Varieties:
Since certain varieties of cherries & plums are so highly susceptible to the black knot disease, a decision will need to be made regarding the practicality of continued preventative maintenance. The long-term yearly multiple fungicide treatments can become too expensive to justify. Tree removal may become the best option when symptoms become more pronounced. Furthermore, the heavily infected trees can attract wood-boring insects that can accelerate tree death.
Some varieties can be tolerant & show little effect from infections. For instance, some of the moderately resistant varieties of plums include Milton, Fellenberg, Damson, Shiro, Brodshaw, Bluefree, Formosa, Early Italian, Methley, & Santa Rose. The effort to manage these trees will more likely be successful.
References:
Black Knot of Plum and Cherry; Pest Management Fact Sheet #5091; Univ. of Maine Coop. Extension; A. Smart, B. Watt, & A. Novak.
Black Knot, Apiosporina morbosa; Wisconsin Horticulture, Division of Extension; Susan Mahr
Black Knot of Prunus; UMass Extension, Landscape, Nursery & Urban Forestry Program; Nicholas Brazee; 2018
Black Knot Not Black; Buckeye Yard & Garden Online; Joe Boggs; 2016
Black Knot; Univ. of Minnesota Coop. Extension; R. Koetter & M. Grabowski
