Millions of California oak trees are dying.

A pathogen causing a phenomenon known as ‘sudden oak death’ has killed millions of oak and tanoak trees in California’s coastal forests since 1995.

It doesn’t affect all trees, but it has a devastating effect on those it does. Traveling through water and wind, the pathogen (Phytophthora ramorum) is a mold that causes tree trunks to crack open and bleed out sap.