1- One of my trees is covered with a dark brown powder, leaves and fruit. The fruit is outstanding nevertheless. What is this and what do I do about it? This tree is planted in the ground.

2- Another potted tree (tangerine) has constant yellow leaves, and loses them easily. Its fruit is tasty as well, but the tree looks fallow. What's wrong?

Thanks in advance.

Answer to number 2. The yellow leaves are caused from lack of magnesium. disolve some epsom salts in water and water your tree.