Streptomyces spp.

(Common and netted scab)

Common and netted scabs are caused by bacteria of the genus Streptomyces, belonging to the Actinomycetes group (heterotrophic bacteria with a filamentous structure and living in the soil).

Two main types of symptoms may be distinguished: common and netted scab, which, accord- ing to recent scientific findings, are considered as two different diseases with separate causal agents seemingly involved and with different responses to climatic conditions and cultivar susceptibility.

- Common scab (raised or pitted scab) is caused by several species of pathogenic Strep- tomyces. In France, at least three species are responsible for this symptom: Streptomyces scabiei, S. europaeiscabiei, S. stelliscabiei. Other species have been described (S. acidiscabies, S. turgidiscabies, etc.) as associated with the common scab symptom, in other parts of the world.

- Netted scab is caused by Streptomyces reticuliscabiei and by some strains of S.europaeiscabiei.

For both types of scab, the soil is the main inoculum source as Streptomyces may persist in the soil on organic matter or on roots or residues of host plants.

• Significance

Common and netted scab are present wherever the potato is grown.

Common scab is a serious issue for fresh ware potato production as it can impair the visual aspect of the tuber.