(4) On any of the following and other similar varieties, rough or thick russeting such as is characteristic of frost injury, when the aggregate area exceeds three-fourths inch in diameter. On any of these varieties any amount of characteristic russeting is permitted whether due to natural causes such as weather or stimulated by artificial means; leaf whips or light limbrubs which resemble and blend into russeted areas shall be considered as russet: Bosc, Clairgeau, Comice, Easter Beurre, Flemish Beauty, Kieffer, P. Barry, Pound, Seckel, Sheldon, Winter Nelis, and other similar varieties.