Black Knot on Plum Trees: Pruning, Spraying, and Saving Your Tree

Disease Profile

Black knot (Apiosporina morbosa) is a widespread, serious fungal disease of plums and cherries. Overwinters in stems; erupts in spring. Produces asexual spores (conidia) early, then sexual spores (ascospores) that cause most new infections. Knots expand yearly, eventually girdling and killing branches.

Control Program

1. Prune 3–4 inches below each knot during dormancy (before April 1).
2. On large limbs to save: cut out infection to wood + 2–3 inches beyond edge.
3. Destroy prunings: Burn, bury, or remove off-site (spores remain active).
4. Remove severely infected trees entirely.
5. Cut wild plum/cherry hosts nearby.
6. Plant resistant varieties if disease pressure high.
7. Fungicide schedule: Dormant lime-sulfur + sprays at budbreak and every 1–2 weeks (especially before rain) until terminal growth stops. Effective products: captan, copper hydroxide, thiophanate-methyl, lime-sulfur, sulfur.

Outcome Depends On

Extent of knots, thoroughness of pruning, and consistent spray program.