Symptoms: A common tuber disease that occurs wherever potatoes are grown, potato scab appears as superficial, dark brown, pithy patches that may be raised and “warty.” These lesions may affect just a small portion of the tuber surface, or may completely cover it. Sometimes the ridged portions are in broken concentric rings.
Potato scab is caused by the bacteria-like organism Streptomyces scabies that overwinters in fallen leaves and in the soil. The organism can survive indefinitely in slightly alkaline soil but is relatively scarce in highly acid soils. It is transmitted to plants by infected seed tubers, wind and water. The organism is also spread in fresh manure, since it can survive passage through the digestive tract of animals.
S. scabies enters through pores (lenticels) in stems, through wounds and directly through the skin of young tubers. In addition to potato, other crops infected include beets, radish, turnip, carrot, rutabaga and parsnips. This should be kept in mind when considering a crop rotation schedule.
Note: S. scabies can survive in the soil for many years in the absence of potato.
Control: Plant resistant varieties whenever possible. We suggest using the russet-skinned varieties, since they have more resistance to the disease. Rotate root crops by planting in alternate locations to limit this disease. Potato scab is most prevalent in dry, alkaline soils. Decrease soil pH by adding elemental sulfur. The disease is controlled or greatly suppressed at soil pH levels of 5.2 or lower. Keeping soil moist during early tuber development may have a dramatic effect on common scab infection. Maintain proper soil moisture for about 2 weeks after the plants emerge from the soil. Avoid overwatering.
Tip: If you will be planting potatoes in soil where tubers have not been grown before or where the area is known to be scab-free, treat seed potatoes with sulfur fungicides to reduce scab introduction.