Black Blotches on Mulberry Trunk: Likely Lichens, Not Disease

Large black blotches on a mulberry trunk are almost certainly lichens—a symbiosis of fungi and algae. They're epiphytes (grow on bark, not in it) and don't parasitize the tree.

Lichen Facts

• Three forms: crustose (flat), foliose (leafy), fruticose (shrubby).
• Thrive in high humidity and clean air.
• Indicate thin canopy (more light reaches bark).
• Slightly detrimental only by reducing bark photosynthesis.

Management

Do not treat lichens directly. Instead:

1. Fertilize with 10-10-10 at 1 lb per inch trunk diameter to thicken canopy.
2. Water deeply during drought.
3. Prune for structure and light penetration.

A denser canopy shades the bark, naturally suppressing lichens. Baking soda sprays (0.4 lb/gal) are sometimes suggested but unnecessary and potentially harmful.