Lumpy Growths on Crabapple: Black Knot or Cedar-Quince Rust

Crabapple tree has lumpy fungal growths on branches for two years. Fruit unaffected. Starts late summer. Tree looks winter-ready by fall.

Two Likely Diseases

1. Black Knot Gall (Apiosporina morbosa)

• Hosts: Prunus species (plums, cherries—ornamental & edible). Not hawthorn.
• Galls: Start as small swellings, become hard, black, elongated, up to 12+ inches.
• Spores: Velvety green on 1-year+ galls.
• Spreads yearly if not removed.

2. Cedar-Quince Rust (Gymnosporangium clavipes)

• Hosts: Hawthorn, quince, sometimes apple/crabapple.
• Galls: Rough, swollen stems, black by late summer, not huge (2x stem diameter).
• Spores: Orange in spring; white cuplike structures on old galls.
• Causes tip blight on hawthorn.

Diagnosis & Action

• Check host: Crabapple = likely cedar-quince rust.
• Prune out galls in winter (dry weather), 4-6 inches below gall.
• Remove nearby cedar/juniper alternate hosts if possible.
• Fungicides (myclobutanil, propiconazole) in spring protect new growth.

See Missouri Botanical Garden link for photos and treatment details.