I couldn't find the reason why but the NC extension center has this to say.
Besides taking care to select only raspberry varieties which have been successful in your area, be sure to buy plants from a reputable nursery. Tissue cultured plants are preferred because they are certified virus free and should be free of Verticillium wilt and crown gall. Dormant canes are not recommended because experience has shown that they are often disease infected. Be cautious about accepting plants from a neighbor's garden. All raspberries are self-fertile and may be planted alone. Plants of red and black raspberries should be separated by 700 ft.
