Common scab is a plant disease of root and tuber crops caused by a small number of Streptomyces species, specifically S. scabies, S. acidiscabies, S. turgidiscabies and others. Common scab mainly affects potato (Solanum tuberosum), but can also cause disease on radish (Raphanus sativus), parsnip (Pastinaca sativa), beet (Beta vulgaris), and carrot (Daucus carota). This plant disease is found wherever these vegetables are grown.
Common scab symptoms are variable and can range from surface russeting to deep pits in root and tuber vegetables. This disease does not usually affect yields, but it can greatly reduce quality of the harvested vegetables and make them unsuitable for sale.
Root and tuber vegetables are susceptible to infection by Streptomyces species as soon as the root or tuber forms, but, because this disease only affects root and tubers, the symptoms are not usually noted until harvest. Dry soils increase disease incidence and severity, therefore proper irrigation can aid in control of this disease. Common scab is suppressed if the soil pH is lower than 5.2, although scab lesions may still form in low pH soils due to physiological stresses or S. acidiscabies.
The first known reference to common scab dates back to 1825 when it was included in John Claudius Loudon's Encyclopaedia of Agriculture. It was not known what caused it then, it was not thought to have a biological cause, but was noted to be associated with soils treated with alkaline materials. In 1884, Worthington George Smith suggested that it was caused by grit in soil irritating potato tubers as they grew. The theory gained much support, as it was known to be more common on light gritty soils. In 1890 however, Roland Thaxter isolated a microbe that could cause common scab lesions, naming it Oospora scabies. Over the years, this species was renamed several times, now being known as Streptomyces scabies. In 1977, the acid-tolerant S. acidiscabies was found to also cause common scab, and then in 1996, S. turgidiscabies was isolated.
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