raised dark bumps on potatoes

Common scab of potato (Streptomyces scabiei)
R.W. Samson, Purdue University, Bugwood.org

Updated: February 20, 2023

Round, raised bumps, or scabs on potatoes

Common scab is a bacterial disease that produces circular, corky, raised lesions or bumps on the tuber or root surface of potatoes. It can also be a problem of beets, carrots, parsnips, turnips, and radish The lesions are tan to brown in color. The disease is encouraged by soil pH above 5.5. The disease overwinters in soil and infected roots and tubers. Spots are superficial and do not affect eating quality once they are removed.


Use resistant potato cultivars and disease-free planting stock. Plant in well-drained soils and lower the soil pH to 5.5-6.0 where scab is a problem (apply 14 lbs. of iron sulfate /1,000 sq. ft. of garden area to lower the pH by ½ unit). Place seed pieces in a paper bag with 2 Tbs. of elemental sulfur and shake to thoroughly coat prior to planting. Keep plants well-watered during tuber formation and rotate root crops to different sections of the garden.