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Extension > Garden > Yard and Garden > Trees and Shrubs > Black knot

Black knot

Rebecca Koetter and Michelle Grabowski

Importance

M. Grabowski, UMN Extension

One young olive green gall and several older black galls

Black knot is a fungal disease that affects several species of Prunus in Minnesota. Outbreaks are common in both landscapes and natural areas. Damage varies greatly between infected trees. Some large shade trees tolerate many galls throughout the canopy with few negative effects. In contrast, galls can result in leaf wilt, leaf, shoot and branch death, and even death of the tree in young or highly susceptible species of Prunus.

Pathogen and susceptible plants

Black knot is caused by the fungus Apiosporina morbosa. Worldwide this fungal disease infects over 25 species of Prunus but in Minnesota, approximately 15 ornamental and edible Prunus species succumb to this disease (Table 1). There are a few disease resistant plum cultivars, but unfortunately none are hardy in Minnesota.

Table 1. Prunus affected by the disease

Most susceptible Less susceptible Rarely affected
American plum
(P. americana)

Nanking cherry
(P. tomentosa)

Amur chokecherry
(P. maacki)

Canadian plum
(P. nigra)
Including: 'Princess Kay'

Pin Cherry
(P. pensylvanica)

Apricot
(P. armeniaca)

Chokecherry
(P. virginiana)
Including: 'Schubert' and 'Canada Red'
Sargent cherry
(P. sargentii)
Flowering almond
(P. triloba)
European bird cherry
(P. padus)
Sand cherry
(P. pumila)
 
European plum
(P. domestica)
Including: 'Stanley'
Sour cherry
(P. cerasus)
Japanese plum
(P. salicina)
Western sand cherry
(P. purmila var. besseyi)
Including: Purple leaf sand cherry
Prunus x cistena
Purple leafed plum
(P. cerasifera)

Identification

Biology

M. Grabowski, UMN Extension

Dead leaves and wilting caused by a black knot gall that has girdled the branch

The fungus overwinters in the galls. During wet periods in the spring, spores are expelled and windblown to infect young green shoots or wounded branches.

Once spores germinate, the fungus grows between the plant cells with no outward signs visible on the plant for several months. During this time the parasite starts growing within the tree and releases chemicals that cause the plant to initiate excessive cell growth and enlargement that results in swollen black galls. Galls are made up of both plant and fungal tissue.

It is not uncommon for the gall to completely encircle and girdle a branch. When this happens the leaves beyond the gall wilt and die. In some cases, the branch and the gall die after spores are released in early spring. If the branch lives, the knot becomes perennial and continues to enlarge, producing new spores every spring. Although the black knot fungus will not cause trunk decay itself, the cracks formed by a trunk infection can provide an entry point for other wood rotting fungi.

Management

M. Grabowski, UMN Extension

Large black knot gall on the main trunk of a Prunus tree.

Site and tree selection

Pruning out galls

The black knot fungus does not systemically infect the tree, but rather only infects the branch at and around the galls. Black knot galls can be removed from infected trees through pruning. This will improve the look of ornamental plants and reduce the amount of fungal spores produced within the tree canopy each spring. Unfortunately, black knot is a common disease of wild and landscape Prunus species in Minnesota. Even with diligent pruning, spores can be blown from infected plants far away and result in new infections. In addition, galls remain very small until a full year after infection. Therefore it may take 2 years of pruning to completely remove all existing infections as young galls are often overlooked.

Fungicides

Fungicides can be used to protect young or highly susceptible Prunus trees from infection. Sprays must be applied in early spring to protect young green shoots. Begin fungicide treatment when flower buds are just beginning to open. Repeat sprays according to label instructions (typically every 7-10 days) until shoots mature or weather is consistently warm and dry. Sprays are most effective when applied before a rain event when temperatures are warmer than 60°F. In order to greatly improve the efficacy of the fungicide application it is important to prune out any existing galls in late winter before applying fungicides in spring.

Before application make sure to read the label carefully! The plant to be treated MUST BE listed on the label or the fungicide cannot be used on that plant. Not all fungicides registered for use ornamental Prunus spp. can be used on edible Prunus spp. For large trees, high-pressure spraying equipment is needed in order to get complete coverage; therefore hire a professional arborist who can safely operate all necessary equipment.

Chemical treatments effective against black knot include fungicides with one of the following active ingredients:

*Always completely read and follow all instructions on the fungicide label.

 

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