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lichen

Q:  What is this pink stuff growing on my oak trees?  Will it kill the tree? 

A:   This growth is called lichen, which is a combination of algae and fungi.  Lichen is not parasitic as some people believe but is an example of symbiosis.  The fungi provide a home for the algae and the algae provides food.  Lichen, along with the green Spanish moss, can often be indicators of healthy air.  In this instance, they both tell us the air in your landscape is healthy and clean.  Your tree may be in decline for other reasons but the lichen and the Spanish moss are not the cause.