There are a number of things that can cause your trees' health to deteriorate, and each condition usually makes itself apparent through a number of symptoms. It's important to familiarize yourself with these symptoms so you can identify and treat them yourself correctly and effectively.
Yellow Leaves- the yellowing of your tree's foliage can be an indicator of numerous problems, so the question is HOW is it yellowing? Is it only present on one side of the tree? Are the leaves gradually changing their entire color or is it forming in splotches?
-If all the leaves are gradually turning from green to yellow, the most likely cause of this is either over watering or under watering. Check the soil at the base of the tree by sticking your finger 1 to 2 inches into the dirt. If it feels moist, then stop watering the tree for a week or two. You can also mix clay into the soil to increase drainage. If the soil is dry, give it a good watering and see how it reacts.
-If the yellowing is contained in splotches or spots, the condition may be more complicated. Yellow spots may be an indicator of a fungus or bacteria attacking your tree. Your best bet is to buy an all purpose fungicide from your local nursery.