I have an oak tree in a pot, around 10 years old, it is 12 - 15 foot high now. I would expect yours to be taller than 3 foot now, is it in a good, very large pot? It may just need more space - think how big oak trees are, and then imagine what size root space they must need. I also would just suggest you check the roots are not waterlogged - something they really do not like at all I have found. I dig each years top dressing of grit into the compost after I have removed the top few inches as far as I can, I then top dress with new JI3, well mixed with vermicuite, then top off with new coarse grit. So far, so good. I'm not keen on sprays, if you don't want to use one you could try washing the trunk with a solution of garden disinfectant, often useful for molds if that is what the cause is. Leaf fall is probably due to stress, either wet or dry, or too small a space, or over heated or chilled - nothing with which it can deal itself as it lives in a pot and you must be all things for it.
We grow many, many things in pots, and they all need alot of attention because of their difficut situations from the plants point of view. Hope it does well for you.