Field maple sapling

I have recently purchased a field maple sapling which I have yet to find a home for so have planted in a pot outside. The pot is just under a foot diameter and over a foot deep. I haven't watered it since getting it but it has rained on it. I noticed the leaves going slightly yellow a few weeks ago but thought it might have been shock but now they are brown and almost furry. I am not sure if over watering or disease. Can you please help me she's some
Light and know if it's given up the ghost! Ta

 

The white spots and furriness on it are powdery mildew and this can make the leaves turn brown. It's probably got too dry as relying on rain in summer to water it just won't be enough.

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Trees in pots will need water usually every day throught the summer.

 

Thanks I've checked the soil and it is sodden pretty much everyday as the joys of North Western weather even in the summer means it rains ALOT! And with it being a plastic pot less water is list than terra cotta. What can I use to prevent the powdery mildew as it appears to be moving to my honeysuckle now? Or is that not how it works?

 

An established tree in a pot will have a canopy of leaves which will shelter the pot from rain.
Are you feeding weekly? Too much water will wash nutrients from the soil.

The top leaves look healthy, it could be that it suffered some sort of drought stress earlier in the year. The usual remedy is to remove all infected leaves and bin them to prevent spread if its fungal.

 

Shall I remove the leaves before they fall or just take them straight off the tree. I am feeding it regularly.

 

Just snip them off leaving the leaf stalk and dispose of them. Leave the healthy looking ones. Next years buds will form soon so you will know alls well. Feed should now be zero nitrogen. I use 0-10-10. This helps bud formation and builds strength to survive winter.