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|
Including: 'Princess Kay'
Including: 'Schubert' and 'Canada Red'
|European bird cherry
|Western sand cherry
(P. purmila var. besseyi)
Including: Purple leaf sand cherry
Prunus x cistena
|Purple leafed plum|
- Knobby swollen black growths called galls grow parallel along the length of stems and branches.
- In early summer, young galls or new areas of growth on the edges of older galls are covered with velvety olive green spores.
- These galls mature by the end of the summer; turning black and hard.
- Black knots are most noticeable during winter because all the leaves are gone and the black knots stand out against the blue sky or white snow.
- Infected branches may distort and bend due to the one-sided growth happening within the gall.
- When galls completely girdle a branch, leaves on infected branches do not emerge or wilt and die in early summer.
- A few galls to hundreds of galls can be within the canopy.
- Large rough black galls that are often cracked can occur on the main trunk of the tree and may ooze sticky liquid.
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.
M. Grabowski, UMN Extension
Large black knot gall on the main trunk of a Prunus tree.
Site and tree selection
- In areas where there are many wild Prunus sp. infected with black knot, avoid planting landscape and edible Prunus sp.
- If only a few infected wild Prunus sp. are present, prune out existing galls or completely remove infected plants to reduce the amount of fungal spores present before planting landscape and edible Prunus sp.
- Choose Prunus spp. that have some tolerance to black knot (Table 1).
- Thoroughly inspect ornamental as well as edible trees and shrubs to ensure they are disease free before purchasing them from the garden center.
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.
- Because black knot is common throughout Minnesota, galls in landscape trees can be tolerated if the disease does not result in wilt and death of leaves and branches.
- In late winter remove all branches with swellings, cracks in the bark or black knots.
- Remove at least 4 inches of healthy wood beyond the black knot.
- Bury, burn or dispose of all branches from a site as spores can still be produced on pruned branches and spread to new areas.
- Cracked and oozing galls on trunks or large branches should be inspected by a certified arborist to determine the structural stability of the tree. Black knot itself will not rot wood but secondary fungi can enter through cracks and result in wood rot and structural instability.
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:
- Thiophanate- methyl
- Lime sulfur
*Always completely read and follow all instructions on the fungicide label.