I must admit that there are several things in the background that bother me, too, and the diagonal line is only one of them. The vertical scratch line with which all my negatives come back and which consequently all the prints carry, too, bothers me way more! It is a downright nuisance!
There were ever so many spider's webs out and about, but that diagonal line is too big for that, isn't it? But what could it be? There was only a field of rye in the background... Hmph...