I've an oak tree, not mature, about 12-14" thick at the base of
the trunk. Quite a lot of the leaves are going brown - the upper
surfaces look normal, except for brown patches of course, but
the lower surfaces are covered in patches of "stuff". Imagine
a miniature umbrella, open, without the handle, with the spike
dug into the underneath of the leaf. Now imagine many of these
per leaf, some mature looking browny spots, some new-looking
green ones, of smaller size. The diameter of the larger spots
must be about 1/6" (just over 4mm). These spots, or scales, are
held onto the leaf by the central "spike", and can be levered
off with the point of my pen-knife.

Here's an attempt at ASCII art, may not come out properly unless
you're using a fixed-width font (poss. not even then!):

################################ - leaf
__|__ _|_ _|_ - things
/ \ / \

Mature -- immature
brown green

The umbrella shape is rather exaggerated.


What sort of nasty are these??? Any help much appreciated!