"Russet" itself is a brownish color, such as that of the eponymous potato.

"Russeting" is the appearance of mottled spots of that color, like on the apple and pear.

"Russeted" might have been more accurate for the fruits, but I suspect there's a phonological reason it's not actually used.

Russets (in apples and pears) tend to have a more crunchy nashi type flesh. The brown skin can be a bit sandpaper and thick. It has a distinctive mouth feel as a fruit all of its own which somehow sets off perfectly with the chalky white crumble which Wensleydale and some Cheshire has.