yellow green mold killing trees in carroll county md
This looks like lichen on your tree trunk and there are many types. They may grow on mature trees and are not a reason for decline. They are considered a good indicator of air quality. Leave them be.
Your ash tree may have declined due to the emerald ash borer, an invasive pest. See our website for more information http://extension.umd.edu/hgic/invasives/emerald-ash-borer
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