Common scab
Streptomyces spp.
Common scab is a plant disease primarily affecting potatoes (Solanum tuberosum) and, to a lesser extent, other root crops. The disease is caused by several species of soil-borne bacteria, with Streptomyces scabies being one of the major culprits. Here's an overview of common scab, including its appearance, symptoms, identification, and management:
1. Appearance and Symptoms:
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Lesions and Russeting: The most noticeable symptom of common scab is the development of raised, corky lesions on the surface of tubers. These lesions may vary in size, shape, and color, ranging from small, slightly raised spots to larger, irregularly shaped areas. Affected areas can also exhibit russeting.
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Cork-Like Structures: The lesions can coalesce, forming cork-like structures on the potato tuber surface. These structures may give the tubers a rough and scaly appearance.
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Color Changes: Lesions can range in color from tan to brown and may become more prominent as the tubers mature.
2. Disease Cycle:
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Soil-Borne Bacteria: The bacteria causing common scab, including Streptomyces scabies, persist in the soil.
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Infection: Tubers become infected during their early development when the bacteria penetrate the young tissues through wounds or natural openings.
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Optimal Conditions: Warm and dry soil conditions, as well as alkaline soil pH, are conducive to the development of common scab.
3. Identification:
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Visual Inspection: Identification involves visually inspecting the tubers for the characteristic raised lesions, russeting, and corky structures.
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Laboratory Testing: For accurate identification of the bacterial species causing scab, laboratory tests, such as culturing and microscopic examination, may be necessary.
4. Management:
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Soil pH Management: Maintaining the soil pH in the slightly acidic to neutral range can help reduce the severity of common scab. Acidifying agents may be used to lower soil pH if it is alkaline.
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Avoiding Excess Nitrogen: Excessive nitrogen fertilization can contribute to the severity of common scab. Proper nutrient management, especially nitrogen levels, is crucial.
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Crop Rotation: Practicing crop rotation with non-host crops can help reduce the inoculum in the soil.
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Resistant Varieties: Planting potato varieties that are less susceptible to common scab is an effective strategy.
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Avoiding Irrigation Fluctuations: Maintaining consistent soil moisture levels and avoiding fluctuating soil moisture can help reduce the risk of scab development.
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Seed Treatment: Treating potato seed pieces with fungicides or antibacterial agents before planting can be part of an integrated disease management strategy.
5. Importance:
- While common scab doesn't generally lead to significant yield losses, it can cause aesthetic issues, reducing the market value of affected potatoes. Severe infections may result in increased waste during processing.
