I have this large tree in my back yard that has about 20% dead branches. The tree is very tall, probably 40+ feet. Most of the branches have this scale on them, and many of the leaves have brown spots. Any idea what this is, or how I can help the tree?
It looks like fungus . Any wood it is on was likely dead before the fungus started growing. Scale are discrete insects , those I have seen on branches are 3 to 4 mm long and many more scales were on nearby leaves. There may be some lichen with the pictured fungus. Once fungus is in a tree I don't know of any fix. I did have an oak about the size of your tree that that did not completely die until 20 years after the first shelf fungus grew out of the trunk.
You didn't mention what kind of tree that is. It looks to me like an ash tree, so this answer is based on that. If that's not correct, please provide more information.
That is probably an ash tree (Fraxinus sp.) dying of infestation by emerald ash borer. There aren't actually any signs of emerald ash borer in the photos, but in infested areas it's a reasonable assumption that any visibly unhealthy ash tree is dying of emerald ash borer. The North American species of ash are in the process of going extinct from this invasive insect. In northern Ohio, most of the living adult ash trees you'll see are infested with EAB.
The "scale" is lichen. Heavy lichen cover is a sign of a very unhealthy tree, or at least that that particular branch is very unhealthy. If my assumption of EAB infestation is correct, the lichen and any other issues are secondary problems. The tree is vulnerable to many kinds of secondary issue, because it's already in a weakened state from the EAB infestation.
Expect the tree to die in the next few years. As individual branches die, they will become brittle and prone to falling, especially in windstorms. Falling branches are hazardous. You probably want to cut the tree down to prevent such hazards.
There are preventative treatments for emerald ash borer. It may be too late for your tree, but you can consult a certified Arborist to see if such a treatment can save your tree. If it can, you will need to continue the treatments regularly for the remaining life of the tree.
I'm pretty sure the white stuff on the bark is a lichen. If so it's harmless to the tree, and is only using the bark as a place to grow.
The leaf brown spots is likely some fungus. Most trees are subject to a bunch of different fungi. In your case, I suspect that it is opportunistic: Your tree is under stress from some other cause.
Add more pictures:
- A pic of the entire tree.
- A pic of the bark where it is not covered with lichen/white stuff.
- A picture of a leaf including it's connection to the twig.
- A closeup pic of the leaf scar where last year's leaf was attached.
- Pics of the cross section of the twigs/small branches when cut with sharp pruning shears.
In each picture include an object to give scale. A coin, a tool, a clothes pin (can be attached to twig...)
Ideally pix are taken with a DSLR. You may want to resize them downward for use on this site, and put the original full size images on a public folder on Dropbox or google drive, or equivalent.