Common Scab (Streptomyces scabies)

Common Scab: biology

The scab bacteria survive indefinitely in soil and invade through lenticels during the early weeks of tuber development. A mixture of cell death and cell proliferation cause the symptoms. Different species of scab bacteria predominate in different soils, but in all cases dry soil conditions during early tuber formation favor scab development.

Once thought to be phases of one disease, these diseases are caused by complexes of closely related bacteria which cause overlapping symptoms. Tubers show unsightly brown discoloration, which may consist of a few spots or complete coverage of the surface. Common and acid scab are indistinguishable and appear as superficial or raised corky spots. Russet scab involves corky reticulate (net-like) lesions and deep pitted scab consists of dark spots up to 6 mm deep.

There is little or no effect on yield or storability, but grade and cooking quality are affected depending on the extent of scab lesions.