I've looked at your pix on alt.binaries.pictures.gardens and as far as I can tell, it looks like cedar quince rust. Take a look at this site and see how it compares to what you see on your hawthorns: http://www.oznet.k-state.edu/path-ext/factSheets/Trees/Hawthorn%20and%20Quince%20Rusts.asp
Suzy O, Zone 5, Wisconsin
size=2>...</FONT></DIV><FONT face=Arial size=2>> Hawthorne, we have four of
them in our front lawn. I'm attaching photos of <BR>> the
situation.<BR>> I noticed a sudden drop of leaves in approximately one
week. Realizing that <BR>> it's just a tad early for the trees to shed
their leaves, I took a closer <BR>> look.<BR>> Discovering what seems to
be some kind of parisitic attack I don't know what <BR>> to make of it.
One of the trees (tree #1 in phototj107) lost most of it's <BR>> leaves, with
a thin layer of leaves all the way at the top. The other trees <BR>>
are covered in these "things". Any assistance will be greatly
appreciated.<BR>> <BR>> -M<BR>> p.s. for photos of the problem, please
see alt.binaries.pictures.gardens <BR>> under the same topic subject.