Diagnosing Red Mounds on Ornamental Crabapple

Symptom Analysis

Two six-year-old ornamental crabapples, 8–9 feet tall. One shows red, corky, blister-like mounds on the trunk and branches; the other is unaffected. Neither tree has flowered this spring. The description matches black knot, a fungal disease (Apiosporina morbosa) common in Prunus species.

Management Steps

1. Prune infected branches at least 6–8 inches below visible galls. Sterilize tools between cuts with 10% bleach or 70% alcohol.
2. Apply fungicide: Use a broad-spectrum fungicide like chlorothalonil (Daconil) according to label directions. Treat the entire canopy of both trees to prevent spread.
3. Sanitation: Remove and destroy all pruned material and fallen leaves. Do not compost.
4. Monitor: Watch for new galls next spring. Repeat pruning and fungicide applications as needed.

Flowering Concern

Lack of flowers may be due to the disease stress, improper pruning timing (crabapples bloom on old wood), or environmental factors. Avoid heavy pruning in late summer/fall. Fertilize lightly in early spring with a balanced fertilizer.

With diligent management, the affected tree can recover and both trees should resume flowering.