Dibotryon morbosum

Dibotryon morbosum is a fungus that affects the genus Prunus. Included in this genus are multiple species of trees and shrubs, though the ones that Dibotryon morbosum infects are Prunus serotina (wild cherry trees), Prunus Persica (peach trees), Prunus Domestica (plum trees), and Prunus cerasus (sour cherry trees).[3] The main symptom of Dibotryon morbosum is its “knot-like” gall structure. These knots can vary in size from anywhere to 0.5-12 in. long, and up to 2 in. wide. This fungus is typically diagnosed by these large black galls at the site of infection. The first signs of these symptoms are noticed during the winter because leaves are not obscuring the view. The first noticeable symptoms are small, light brown swellings. The next season, these swellings will turn dark green and have a velvety texture. The green swellings will darken and harden into the large black knots. These swellings often start as green in color during the beginning of the summer season and become black at the end of summer.[4]

Dibotryon morbosum has a relatively simple disease cycle. In the spring, after overwintering in a previous host, the fungus produces ascospores, which are stored in a fruiting structure known as the pseudothecia. These ascospores then get dispersed by the wind and rain until they find a susceptible host.[5] They typically infect their host on wounded tissue or shoots. The infection is not systemic but does grow and spread from the initial infection site. Symptoms are typically not noticeable in the season of initial infection, as the fungus grows inside the host. Throughout the summer, conidia are produced which also get dispersed by wind and rain. Conidia are asexual spores that help the fungus to spread within a given growing season. The conidia help the fungus germinate and produce other generations of the fungus, after, the conidia also helps the spores disperse by releasing them and allowing them to disperse through the wind.[5] The fungus then overwinters within the shoots and tissue of the host. It then produces ascospores in the spring to begin the cycle anew, but only after it has grown on its host for two winters. It will not produce ascospores the first season after initial infection but will continue to produce asexual conidia

Dibotryon morbosum produces pseudothecia, fruiting structures that are embedded in the black stroma on the surface of the gall.[6] In the spring, two winters after initial infection, the fungus produces sexual spores called ascospores. The ascospores mature during the early spring of the infection's second season and are forcibly discharged into the air during rain events. The spores are distributed short distances on wind currents and through rain splashing. The anamorph, or sexual stage, produces abundant olive-green conidia during the summer on the surfaces of one-year-old knots.[6] The infection capabilities of the conidia are quite limited. Therefore, management strategies are focused on ascospore development and infection processes.

The fungus Dibotryon morbosum overwinters in the knots and once the ascospores are released. The spores are released during the wet periods of spring. The wind and rain carry these spores to infect young saplings or wounded branches.[7] The fungus favors warm and wet weather with any temperature within 60-80 degrees Fahrenheit as it is the most ideal for dissemination, germination, and infection of new plant tissue. Rainfall is also significant because it causes the spores to be released and begin infecting new plant tissue. The splashing of the rain helps transfer the ascospores along with air currents.

There are several ways that Dibotryon morbosum is managed. The first way to manage this pathogen is to choose strains that are genetically resistant. There are several resistant species of the genus Prunus that can be used, such as Prunus maacki, Prunus armeniaca, and Prunus tribola. Cultural management can also be used to help prevent Dibotryon morbosum. Removing the source of inoculum, ascospores, by pruning plants can be effective in managing this fungus. Pruning the branches should happen before spring as that is the time the buds break.  It is also important to consider where Prunus species are planted. Areas that are known to have had issues with Dibotryon morbosum should be avoided because the black knot is a widespread fungal disease and will easily attack new growth and cause deterioration of plant and fruit growth.[8]

Chemical management can also be an effective way to manage Dibotryon morbosum. Fungicides can be used, but they are only recommended for use in severe cases and are often not very effective unless used with other cultural methods. Additionally, fungicides will only be effective if the source of inoculum is no longer present. Another way to manage this disease is by pruning the branches. Winter is the best time to look for galls since there are no leaves to obscure. By removing the galls the ascospores won’t mature and spread to healthy tissues. Fungicides are able to be used but are only recommended for sites with valuable trees or very severe cases of the black knot. The fungicide will only work as a protectant if the cultural practices that were before mentioned are being done as well.[9]

Black Knot disease is a very common and harmful fungus. This disease is not well maintained in the wild and orchards. Black Knot is able to spread so well because its spores are carried throughout nature during rainfalls and are also carried by the wind. This is an important pathogen because it is very common and can be easily spread. If not taken care of, black knot can cause detrimental losses in harvest.

• Index Fungorum entry
• USDA ARS Fungal Database
• West Virginia University Black Knot page
• Ohio State University Extension, Black Knot Fact Sheet