Both the common and powdery scab pathogens are sometimes described as fungi, but in fact Streptomyces species are more closely related to bacteria and Spongospora subterranea f. sp. subterranea is related to the slime moulds.
Both organisms exist in the soil, either free-living (Streptomyces spp.) or as spores (S. subterranea f. sp. subterranea). They invade the surfaces of potato tubers and the plant responds by growing corky scabs, which actually limit the spread.
Powdery scab is worse under wet conditions because its spores germinate to produce infective swimming spores that need water for infection. By contrast, severe attacks of common scab can occur if the soil is dry during early tuber development.
Light attacks of both diseases are only superficial and do little to affect eating quality, but they are commercially important because cosmetic damage lowers the value of the crop. Severe attacks can lead to cracking of the skin and rotting of the tubers.
Very severe attacks, especially of the cankerous form of powdery scab, may be mistaken for infection by the serious disease known as potato wart. Wart is now very rare in the UK, but is notifiable and if it is suspected, the relevant plant health authority should be informed. Contact details can be found on the UK Plant Health Information Portal.