Funny marks upon this tree, any IDs, could it be a new tree disease?
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The tree actually isn't a sallow, it's a Goat Willow.
Longworth Traps or The Bryophyte Club!
NBN awards 15 winner.
... sallows are any of several small species of willow, especially the grey willow )Salix cinerea). Many people include Salix caprea, Salix aurita, and hybrids between the 3 species, as well as Salix cinerea within the concept of sallow.
If this were pure Salix caprea it is called "Goat Sallow" as well as "Goat Willow".
However, while it is probably not possible to be sure from the photo, it looks a bit more like the hybrid with Common Sallow to me (Salix caprea x cinerea, = S. x reichardtii). This hybrid is often more common than pure S. caprea in urban areas or away from natural habitats.
Braun's 1995 European monograph of the powdery mildews records Uncinula adunca on Salix x reichardtii, in the listing under var. adunca, though the records may well have been just for the species. I note that according to the BMS database, U. adunca is now transferred to Erysiphe.
Lat/Lng: 51.1436, -0.0796
OS grid ref: TQ344400