Lichens on Mountain Ash: Harmless Epiphytes, Not Disease

Those Odd Growths Are Lichens, Not Pests

A mountain ash has brownish-purple, sectioned nodules with yellow spikes, plus a dark spot of peeling bark. Nearby magnolia had scale last year. The owner worries about spread and harm.

Diagnosis

The nodules are lichens—symbiotic algae/fungi that grow on bark, not parasites. They indicate a thin canopy but don't cause decline. The peeling bark spot may host a decay fungus fruiting body; it won't kill the tree but can hollow it over decades.

Care

• Improve tree vigor: fertilize with 10-10-10 at 1 lb per inch trunk diameter.
• Dense canopy shades limbs, reducing lichen growth naturally.
• Magnolia scale is unrelated.

No treatment needed for lichens; focus on overall health.