Unfortunately, the only person I found working on powdery mildew of
maple was a Chinese molecular biologist who didn't understand a word I
was asking him. So instead I'll tell you about oak wilt, a disease I
was vaguely aware of, but which is a hot topic in Austin because trees
are dying of it and the public feels that we scientists are just
twiddling our thumbs doing nothing.

Oak wilt was first observed in Wisconsin in the 1940's, but now
inhabits the central US from Texas to Minnesota. It is caused by a
fungus related to the Dutch Elm disease pathogen, so people were afraid
it would turn out to be as devastating as Dutch Elm, which thankfully
it hasn't. Like Dutch elm disease, it is carried by beetles. Beetles
are attracted to the fungus because it has a heavy, sweet odor; they
get spores on them and then fly to another tree, spreading the disease.
The fungus, when it comes in contact with the xylem (wood) of the
tree, produces tiny spores that can travel in xylem tubes and within a
matter of months infect the entire tree, clogging the xylem and killing
it. There is no treatment for the disease except injection of a
systemic fungicide, but a researcher in the symposium I attended said a
tree would probably have to be treated for the rest of its life to save
it. For an oak, we might be talking 400 years!

Applicability to bonsai? Well, nothing to worry about unless you grow
oak bonsai and live in an area with infected oaks. If you do, however,
you need to be very careful pruning your tree; the beetles that carry
the fungus can smell a fresh wound within minutes of it being made and
will fly over to it. I would recommend cut paste for even small
pruning wounds.

Nina