[dave9t5]

I thought about moving air too... it's quite common in landscape with warm air (BTW, it was not warm, about 3-4°C - but I think we can have turbulence even with cold air and full sun).

 

But... just in such a small part of the image? Furthermore, in all cases of moving air that I'm experienced with I see a sort of twist effect, while branches, while blurred, are still "straight".

 

I'm seeing blur all the way to infinity in that streak.  But...there is a double image in one of the trees as well.   If not for the double image, I might suggest to check if there is a small smudge on the rear element (which might account for such a localized blur).