Blossom wilt.

Have had a bit on the Sturmer pippin for the last few years, but now quite a bit on the James Grieve and first appearence on one Bramley this year.  Not clear what to do about it (if anything), as we still seem to get a crop of apples.  Once the apples start to develop they seem unaffected - which is why we were initially inclined to think it was linked to unfertilised blossom (maybe due to fewer pollinators).