Maple Tar Spot and Crown Thinning: Management and Tree Vigor

Understanding Tar Spot

Black, raised spots on maple leaves are tar spot, caused by fungi in the genus Rhytisma. While unsightly and sometimes causing early leaf drop, tar spot rarely threatens tree health. The fungi overwinter on fallen leaves.

Why the Crown Is Thinning

Progressive crown thinning and lichen on the trunk suggest chronic stress—not tar spot alone. Contributing factors:

• Repeated early defoliation reduces energy reserves.
• Lichen indicates slow growth and thin canopy (more light on bark).
• Possible root issues: compaction, girdling roots, or drought stress.

Management Practices

• Rake and destroy fallen leaves in autumn to reduce spores.
• Mulch leaves if composting—cover pile until spring.
• Fungicides (mancozeb, triadimefon, copper) only for severe, recurring cases; difficult on large trees.

Boosting Tree Vigor

Apply 10-10-10 fertilizer at 1 lb per inch of trunk diameter, scattered to drip line, watered in. Best before rain. This supports recovery from defoliation and improves canopy density, which naturally suppresses lichen.

Long-Term Outlook

With consistent sanitation and vigor improvement, the tree can recover. Tar spot fluctuates yearly; a dense canopy is the best defense.