Botryosphaeria. It requires an injury to enter (apparently)
and
a wound from removing a branch or twig -- or even severe
bendings -- seems to be enough.

Botryosphaeria is one of the common canker-causing fungi. It
is not a vascular disease- it can only spread externally through
wounds, although an existing canker will expand and eventually
girdle the branch. It is more severe on stressed plants.
There's nothing to do except excise the canker, and if a bonsai
is permanently marred by doing that, well, it's off to the
compost pile.

If you are having a big problem with it in Florida, it must be
due to some weather condition: drought? Hail? that is
exacerbating it.

Drought. Landscape trees are getting it though damage by mowers
and string trimmers, etc. Something like 45% of the Drake and
lacebark elms in Tallahassee are going or gone. No loss IMHO.

But this may no be what I'm seeing on my bonsai elms. (It's not
leaf spot, either, as far as I can tell. The leaves turn
freckled brown, then crispy, then black -- like they have been
burned with a blowtorch.)

Jim Lewis - - Tallahassee, FL - Nature
encourages no looseness, pardons no errors. Ralph Waldo Emerson

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