Disease of oaks
I have two oak trees (different species) that are probably 50 or more feet high and probably 3 feet in diameter a few feet above the ground. I have lived here 25 years and they were almost the size they are now 25 years ago. These trees were fine last year and I think they were fine this past spring, but recently and rapidly most of the leaves have turn completely brown. There still seems to be some green left lower down on each tree. Interestingly, the leaves are not falling. I had a tree remover look at them and he said he had some other cases and he had noticed that the ones he had removed had a "green powder" on the bark and he thought it was an airborne infection of some kind. A few small branches that had fallen from my trees also show a mottled pattern of a green mold-like substance. The branches are light in weight and very brittle. There are also some small holes which may have been from woodpeckers, but not anything serious. I am assuming that both trees are dead and I am making plans to remove them. However, are they dead? I do not want to lose the trees, but if they were to fall, either one could hit the house. Any ideas?
We are glad you contacted us.
There are many reasons that trees could brown out, including drought, scorch, disease, root rots, etc. Finding the main cause and to what extent the tree is declining is important for you.
Here is our IPM Shade Tree publication which will show you some common causes for different symptoms: https://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG61_IPM_Series_Sha...
The 'green powder' and mottled mold-looking things you describe are lichens. They are totally harmless growths (a symbiosis of a fungus and an algae) which grow on slow growing or dead wood, and not disease. It would be pretty normal to have some dead branches in trees of this age, and again normal to see these growths.
Since you say the trees are a hazard were they to fall, we recommend contacting a tree health specialist called an arborist certified by the International Society of Arboriculture for an on-site evaluation. ISA -certified arborists can advise you on what is going on and what to do. Some will come for free to do the evaluation, but ask. You can search for a certified arborist in your area at www.treesaregood.org.
You should also take a look at our publication which helps you decide when to remove a tree: https://extension.umd.edu/learn/how-do-you-decide-when-remove-tree