The first time I witnessed Black Knot, I was conducting a tree survey for a family that was putting their house up for sale. I started in the front and made my way around to the rear yard where the family’s three children where playing. I rounded the corner to see a very large plum tree that had a sever case of black knot. When the children saw me looking at the tree, they asked me, “Are you here to clean the poo off of our poo tree?” Ah, out of the mouths of babes….
Black Knot of plums and cherries is a common and serious disease throughout the United States. The disease becomes increasingly worse during each growing season and unless effective control measures are taken, it can stunt or kill the tree. The black knot fungus can infect American, European, and Japanese varieties of cultivated plums and prunus. Sweet and tart cherries are also affected by the fungus, but are generally less susceptible than plum or prune. Sometimes, it may also infect apricots, peaches, and other Prunus species.
The fungus overwinters in the galls. During wet periods in the spring and when the buds of the tree swell, 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 fungus starts growing within the tree and releases hormones that cause the plant to initiate excessive cell growth that results in swollen black galls. The galls contain both plant and fungal tissue.
It is not uncommon for the gall to completely encircle and girdle the branch of the tree. Usually when this occurs, the leaves beyond the gall wilt and die.
Sometimes, the branch and the gall die after spores are released in the 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.
- Prune out galls during the winter. Cut should be approximately 10” inches away from the gall.
- Fungicides should be applied when the host plant starts to bud, or when Magnolia x soulangiana is in pink bud to early bloom. Continue to spray every 7 – 10 days until there has been about 1 1/2” inches of growth or when Magnolia x soulangiana is dropping its petals.
- Chemical treatments effective against black knot include fungicides with one of the following active ingredients:
- Thiophanate- methyl
- Lime sulfur
© The Naturarian