Common scab – Streptomyces scabies

Symptoms   Symptoms range from almost circular to irregular, angular lesions on the surface of tubers.  Actively growing tubers are infected through young lenticels. Lesions may be superficial, or penetrate to up to 5mm depth; in some cases lesions may have a raised appearance.  Lesions may coalesce to cover a large part of the tuber surface. The type of lesion is believed to be determined by the aggressiveness of the infecting strain, the scab resistance of the potato cultivar and the environment.

Sources and Factors Favouring Development

Infection occurs from soil in which Streptomyces (a form of bacteria) is widely distributed. Planted infected seed tubers do not provide a significant source of infection for the daughter crop. Applying lime or other similar soil amendments to increase the pH of the soil can also increase the levels of common scab in.

Controls       Statutory tolerances for classified or certified seed stocks provide effective limits to infection. The disease is usually more severe during dry warm seasons (soil temperatures above 22° Celsius).  Controlling soil moisture (if irrigating is possible) to just below field capacity for a period of four to six weeks, beginning at tuber initiation (swelling of the root tips). Potato varieties do vary in terms of resistance to Common scab but none are immune and severity and incidence depend on the strain of Streptomyces and soil moisture conditions at tuber initiation. There are no chemicals that have been approved for controlling Streptomyces strains.