Black Knot
Apiosporina morbosa (Dibotryon morbosum)
Overview
Black knot (Apiosporina morbosa) produces hard, coal-black galls on plums, cherries, and other Prunus. Galls enlarge over two seasons, girdle limbs, and deform canopies - a conspicuous problem in orchards and wild plum edges that re-infect cultivated trees. Black Knot spreads mainly by spores on wind, splash, tools, and crop debris. Humid, crowded plantings and overhead watering favor outbreaks in home gardens and food forests. Causal agent: Apiosporina morbosa (Dibotryon morbosum). In regenerative gardens, prioritize sanitation, airflow, and unbroken soil cover over calendar spraying, and intervene early when symptoms appear.
Symptoms
Olive-green swellings that mature into elongated, rough black galls on twigs and branches. Limbs beyond galls may die. Heavy infection weakens trees and ruins form. Compare several plants and note whether symptoms spread after rain, insect feeding, or wet foliage. Photograph undersides of leaves and cut stems when wilt is present to refine the diagnosis. Rule out simple drought, fertilizer burn, and herbicide drift before assuming a pathogen - abiotic injury often mimics disease.
Environmental Conditions
Humid
Prevention
Choose resistant plum and cherry types when available. Scout and prune knots every winter. Clear wild Prunus within wind range. Keep canopies open so bark dries after rain. Mulch to reduce splash, clear season-end debris, and avoid working plants when wet. Design beds for airflow and diverse plantings that support beneficial insects without creating permanent humid pockets around susceptible crops.
Management
In dormancy, prune at least four inches below each gall and destroy prunings (burn or landfill - do not leave piles). Lime-sulfur dormancy sprays reduce new infections. Remove severely knotted trees and nearby wild hosts when practical. Prefer drip irrigation, morning watering, and open spacing so leaves dry quickly. Disinfect pruners between cuts on woody plants. Use copper, sulfur, or biological products only as labeled protectants when pressure is high - they rarely reverse advanced infections alone. Destroy (do not casually compost) highly infectious debris.