Common scab – Streptomyces scabies, bacteria
"Common scab is often characterized by a corkiness of the tuber periderm, but symptoms are extremely variable. Lesions may be very superficial or may penetrate up to I/a inch into the tuber surface and have been described as russetted, slightly raised, slightly pitted, or deeply pitted. Up to 100 percent of the tuber surface may be affected by lesions, which are light brown to dark brown. Insects are attracted to affected tissue and may enlarge lesions. Symptoms of common scab may be confused with those of powdery scab, Rhizoctonia russeting, or insect damage. This disease usually occurs in soils with pH values higher than 5.2 and when the soil is dry during tuberization. Acid–tolerant Streptomyces spp. that cause symptoms indistinguishable from S. scabies have been reported."
"Although common scab does not spread in storage, infected seed can lead to infection of daughter tubers and contamination of soil. Avoid heavy manuring, especially with poultry manure. Controls include clean, certified seed, resistant varieties, and fungicide seed treatments that are active against S. scabies"
Information provided by: Zitter, T.A., Loria, R. "Vegetable Crops", Cornell University Vegetable MD Online.