Tubers from plants infected in the current season develop fine lines of discoloration in the tuber. Spindle Tuber Spindle tuber is a serious disease of seed potatoes caused by the spindle tuber viroid. Plants infected with spindle tuber viroid are upright, dwarfed and much thinner than normal vines. The stems are often more branched and branches form very sharp angles where joined to the stem. Affected tubers are dwarfed and are usually narrow and spindle-shaped.
Eyes are numerous and the tubers are often cracked.
Spindle tuber is spread by insects such as aphids, grasshoppers, Colorado potato beetles and flea beetles, as well as seed-cutting knives, planters and infected seed. Mosaic is caused by potato viruses A, X, S and Y, alone or in combination. Infected plants have yellow mottled leaves that may be wrinkled, distorted or reduced in size. Mosaic viruses are spread by insects, infected seed-pieces, cutting knives and by contact with infected plants. Witches' Broom Witches' broom of potatoes is caused by a virus-like mycoplasma.