Iris,
I've had brown spots on buttonwood once outside...too cool - wet?
I haven't even taken my buttonwood outside yet, it's still too cool.
I've found that buttonwood roots regenerate best after a re-pot if they
are kept consistantly warm day and night with good light.
This is why I re-pot in early spring inside.
Pot? Tough to tell without a picture.
I'm starting to order my pots from Japan for next year, so I'm going
through the tokoname catalogue right now.

Regards,
Alan

Blue Mountain Bonsai
Ontario, Canada
Zone 6