An Ugly disease of the Rose Family
Black tar-like, knobby swellings on the stems of plum and cherry trees are caused by a fungal disease called black knot (Apiosporina morbosa). This very prevalent disease affects members of the rose (Rosacea) family especially plums in the Pacific Northwest.
Causes: Pruning increases the chance of infection as the wounds are open to airborne spores. Weakened trees are vulnerable so provide good drainage and water during periods of drought. Stagnant air, high humidity, mild wet conditions also encourage this disease.
What to do: In winter, when plants are dormant, remove the growths by cutting the stems back to healthy wood at least 6 to 8 inches beyond the black knot. It’s imperative to disinfect pruning tools after each cut with 70% rubbing alcohol.
Destroy all infected parts from the plant and from the ground immediately. It’s recommended to burn all contaminated plant parts where possible. Severely infected trees should be removed. Avoid replacement plants that are members of the rose family: apples, apricots, cherry, nectarines, peaches, almonds, pears, raspberries, strawberries and roses.
Treatment: Spray with dormant oil/lime sulfur while plants are still dormant in late winter. Treat all members of the rose family in your garden with dormant oil to reduce the spread of the spores. Also apply it to the soil around and under the plants. On infected trees, apply neem oil when buds open. Repeat every 7 days.