Hi and welcome
If you would fill in your profile with your location we'll be able to give you better ideas as to why your tree is doing this.
It looks and sounds more like a watering issue to me, although fluctuating temperature would make that problem even worse. Have you pulled the tree out of its pot and had a look at the roots? A plant like that in a pot needs deep watering, usually with a hose or watering can and enough so that water comes out the bottom of the pot. As it's probably grown a lot over the summer, it also would need an increasing amount of water. So if it's just getting sprinkled with your lawn sprinklers, it's possible that the root ball has dried out too much, having just been wetted on the surface.
It's also possible that the tree has filled the pot up with roots, and that also may cause it to need a lot more watering than it's getting. So on cool days it might be ok, but on a sunny warm day, it would repeatedly wilt in the heat of the day and the leaves would die off after a few days of that.
Btw, Daconil is a very toxic fungicide both for you and your local birds, critters and insects and should be a last resort when you KNOW there is a fungal issue. Be careful about "using a sledge hammer to swat a fly" here.
