Your tree is infected with black knot. Black knot is a common and sometimes severe fungal disease of stone fruit trees in the Michigan and similar climates caused by the fungus Apiosporina morbosa. The pathogen causes “knot-like”growths on the branches and limbs. Limbs affected by black knot become progressively weaker and less productive. A severely infected tree can become worthless in a few years.
Black knot can be confused with crown gall, caused by the bacterium Rhizobium radiobacter (formerly Agrobacterium tumifaciens) which also can occasionally cause tumor-like swellings on above ground plant parts. Black knot galls are generally darker and spread more readily than crown gall.
Black knot can be severe on plum and wild cherries and is an occasional problem on cherries, peaches, and apricots and many ornamental trees such as flowering plum, sand cherry, flowering almond, black cherry, and pin cherry. The plum varieties Stanley and Shopshire are highly susceptible, Methley, Early Italian, and Fellenburg are moderately susceptible, Shiro and Santa Rosa are moderately susceptible, and President is resistant to black knot. The disease is favored by moderate temperatures and wet conditions during active shoot growth.
The fungus overwinters in black knots and non-symptomatic infected wood. In the spring, beginning when the plum growth starts, spores in the black knots are spread by wind and rain. Actively growing plum shoots are susceptible to new infections during extended wet conditions and temperatures at least 43 F. Infected limbs will eventually develop knots, beginning in a few months.
Occasionally, an infected branch may not show any swelling during the year of infection. Typically, knots showing up in spring are due to infections in the previous year.
To control black knot, careful pruning to remove knots is critical, at least twice per season and whenever knots are noticed. Cut knots and at least 5 or 6 inches of symptomless limb next to each knot. Disinfect pruning tools with either a 10% bleach solution or isopropyl alcohol between making cuts. Remove knots from orchard and bury or burn. Check orchard boundaries for wild plums and cherries for knots and remove knots or entire trees.
Several treatments with fungicides specific for black knot may be needed during active limb growth, from bud break until several weeks after bloom. Fungicides with best activity against black knot include Indar and chlorothalonil. The fungicides captan and copper have fair to poor activity. Check product labels for further information. Once black knot is established in planting, aggressive pruning and fungicide applications are needed over several years to restore it to a disease-free status.