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Black Knot Disease on Plum Tree: Fruit Impact and Control

Effect on Fruiting

Black knot (Apiosporina morbosa) girdles young branches where fruit forms, indirectly reducing yield. Bloom is unaffected, but if flowers appear and no fruit sets, poor pollination may be the issue (need compatible pollinizer).

Control Measures

Prune out knots: Cut 4–6 inches below visible galls in winter. Destroy prunings.
Fungicide: Apply captan or chlorothalonil at bud break and petal fall.
Sanitation: Remove wild plums/cherries nearby (alternate hosts).

Reference

NC State Extension: Black Knot of Plum.

Trees: Black Knot Disease on Flowering Plum: Pruning and Prevention; Black Knot on Plum Tree: Prune, Sanitize, Fungicide for Prevention; Black Knot Disease on Plum: Pruning and Fungicide Protocol; Black Knot on Ornamental Plum: Organic Atomic Grow Treatment; Flowering Plum Black Knot: Identification and Control; Black Knot on Plum Trees: Pruning, Spraying, and Saving Your Tree; Lumpy Growths on Crabapple: Black Knot or Cedar-Quince Rust; Black Knot on Plum Trees: Fruit Safety and Disease Management; Plum Tree Pruning for Fruit Production; Managing Severe Black Knot on Cherry Trees

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