Question
My father has a lemon tree in his back yard in So. Cal.  the soil seems to be fertile enough.  However the leaves have got what looks to be a fungal disease, on some of the leaves there looks like a slimy substance and after a while holes begin to appear.  It almost looks as if snails were chewing on the leaves but after close inspection none were to be found. I believe the tree is far beyond its infant stage but I have no idea of it's age. Would know of this disease and what can be done to rid of it  

Answer
Jack,

From what you are describing, I would have to guess that it is a citrus saw fly larvae or possibly slugs. Both of these produce the same type of damage. Check the undersides of the leaves for anything for egg masses or larvae (small slug like creatures). If none ae found, check the plant at night when slugs are known to feed.

There are two separate treatments for these  pests. Slugs require treatments of mollusicide and saw flys will be controled with a broad spectrum insecticide.

Good luck.
Ed Gulliksen