Folks,

Our lovely lemon tree is in trouble... We live in Northern California. Starting mid-last summer our tree began yellowing. We've called an arborist but the advice has been pretty useless. Can anyone in gardening StackExchange land help save it?

Here are the symptoms:

1. Most leaves are turning yellow and very few leaves at top of tree.

2. The fruit is smallish relative to past season.

3. The bark is peeling starting from the base of the trunk and going up.

We have supplemented with Iron/Magnesium solution per directions (1% by volume) two times over two months, total volume of 2.5L poured around base tree and roots. This has not seemed to help the color.  I measured the soil pH and best as I can tell from the kit it seemed normal within range for citrus trees. Also, there seems to be a green mossy color on the branches.  We're not sure if this has always been there or if it's getting worse.

Questions for StackExchange include.

1. What is wrong with our tree?
2. What is causing the bark to peel?  Is this "collar rot"?  Something else?
3. Are the yellow leaves related to the peeling bark at base of tree?
4. How can we fix the bark, the leaves. Basically, how do we save the tree?

We're not gardeners but we love this tree... please help if you can!
Any advice, questions or treatment options?

Thanks, StackExchange.
CuriousGuy

PS I am new user to Gardening StackExchange so I can't post pictures yet...I will edit this to include pictures of each of the items above **as soon as my reputation goes above 10**.  Upvoting the question may do it. I'm not sure how the point system works here.