Common scab
Common scab is a plant disease of root and tuber crops caused by Streptomyces species, specifically S. scabies, S. acidiscabies, and S. turgidiscabies. 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.
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