Tar Spot
Rhytisma acerinum, Rhytisma punctatum
Overview
A fungal disease primarily affecting maples, causing raised, shiny black spots on leaves that resemble drops of tar. Generally cosmetic but can cause premature leaf drop in severe cases. Tar Spot 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: Rhytisma acerinum, Rhytisma punctatum. In regenerative gardens, prioritize sanitation, airflow, and unbroken soil cover over calendar spraying, and intervene early when symptoms appear.
Symptoms
Raised, thick, shiny black spots (1-2 cm) on upper leaf surfaces resembling tar drops. Yellow halos may surround spots. Heavy infections cause premature leaf drop in late summer. 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
Rake and destroy fallen leaves in autumn to reduce inoculum. Improve air circulation. Generally does not require chemical control. 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
Rake and compost or destroy fallen leaves. Fungicide application is rarely warranted. Maintain tree vigor through proper watering and fertilization. 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.