Shademaster Locust Lichen Growth: Cosmetic, Not Pathogenic

Those "Moldy" Branches Are Actually Lichens

A 20-year-old Shademaster locust (Gleditsia triacanthos 'Shademaster') with thin canopy and gray-green "mold" on branches has lichens—not a disease.

Lichen Facts

• Symbiotic organisms (fungus + alga/cyanobacteria).
• Grow on bark, rocks, fence posts, slow-growing plants.
• Not parasitic—use tree only for support.
• Indicate good air quality and stable bark surface.
• Often increase on stressed/slow-growing trees (more time to colonize).

The Real Issue: Canopy Thinning

Lichens are a symptom, not the cause. The 50% canopy loss and dead branches are the problem. Causes for locust decline:

• Age: 20 years is mature for 'Shademaster' (often 20–30 yr lifespan).
• Cankers: Thyronectria canker causes branch dieback.
• Root issues: Girdling roots, compaction, construction damage.
• Borers: Locust borer (Megacyllene robiniae) tunnels in trunks/branches.
• Environmental stress: Drought, wet soils, herbicide exposure.

Recommended Actions

1. Ignore the lichens—they're harmless and beautiful.
2. Hire a certified arborist to diagnose the canopy decline. They'll check for cankers, borers, root health, and structural issues.
3. Prune deadwood for safety and aesthetics.
4. Water deeply during drought; mulch to dripline.

Locusts are relatively short-lived urban trees. At 20 years, plan for eventual replacement while maximizing current health.