Impossible to know without photos of your trees and soil. Could be either. I would suggest you check the very bottom of the pots by inserting a stick down to the very bottom of the pot. If it comes back very wet, you have your answer. It could be that you've got too dense potting mix, and it is retaining water. If this is the case, a re-potting with a more porous potting mix might help the situation. Since these trees are outdoors all year long, you might consider a slightly modified version of the "Gritty Mix" or "511 Mix" recipes on this list, adding a little more fine particles (peat based potting mix).

Patty S.

The two trees are potted in a well drained Cactus Mix.

Eureka Lemon

Eureka Lemon

I think your pot is too small. I grow citrus in wine or whiskey barrel planters which stay outside all year - I have terrible soil, or I would put them in the ground, but the roots need room to expand. Every other year, I take the trees out of the planters and add more soil to the bottom. I use a similar potting soil to yours. The soil in my yard is dense heavy clay that does not drain, and so most plants drown, they do very well in pots and planters. I also have a Eureka lemon because it makes lemons at a different season from my Meyer lemon, but I freeze any lemons whole that I cannot use right away, for the off season.

Lars

If a citrus tree is allowed to dry out for more than a day, you will not see the damage until you water it again, which may cause confusion A citrus tree that has been left dry will lose leaves when watered. The longer the citrus tree is left in dry soil, the more leaves it will lose when you water it. This is confusing because most plants lose leaves when they dry out. Citrus trees lose leaves after you water them once they have dried out.

If your citrus tree is getting too much water, meaning that the drainage is poor, the leaves will yellow and then fall off.

Just reading this thread, as I am experiencing something similar with my Eureka tree. I bought it May of 2013 and live in northern Ontario. Climate is very cold here around -25 daily in the winter and averages 25 degres Celsius in the summer. My tree is dropping all of its leaves and the branches are all turning brown. It is still in the same soil and pot it came with and I had intentions of repotting it this coming spring, but wondering if I should do it now. Will it survive? Please help I have no idea what I'm doing!