This tree was wintered outside last winter(buried + mulched). Seven
other Q. robur were wintered in a garage and show no sign of mildew.

That shouldn't have made a difference, unless the other trees stayed
in the garage during the spring infection period, or unless the
buried tree was mulched in infected oak leaves.


I suppose I could defoliate the tree either gradually or completely(this
hasn't been done for a few years on this tree).

Powdery mildew isn't harmful on oaks (it's more unsightly than
anything else); what you should do is to wait until fall and remove
the leaves as they senesce; those yellow dots are the over-wintering
stage, and will be infective next spring.


Any ideas on a fungicide or baking soda solution?

Too late for this year; next year you could treat the leaves
prophyllactically, but home-recipe baking soda won't work on oak
leaves; you should buy a formulated baking soda fungicide with
adjuvants to allow better penetration, or find a fungicide labeled
for powdery mildew on ornamentals.

Nina, who just took her applicator's license test, hence use of words
like "prophyllactic" and "adjuvant".

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