Potato scab is a common disease affecting potatoes worldwide. It is caused by several species of soil-inhabiting bacteria, primarily Streptomyces spp. and more rarely, the fungus Spongospora subterranea. Here’s an overview of its symptoms, recommendations, causes, and preventive measures:
Symptoms:
- Scab Lesions:
- The most characteristic symptom of potato scab is the formation of superficial corky or rough lesions on the surface of potato tubers.
- These lesions can vary in size, shape, and color, ranging from small, slightly raised spots to larger, more extensive patches.
- Lesions are typically brown to tan in color and may have a scaly or cracked appearance.
- Irregular Shapes:
- Scab lesions often have irregular shapes and may coalesce to cover significant portions of the potato surface.
- In severe cases, lesions can cause distortion or malformation of the tubers, leading to reduced marketability.
- Flesh Discoloration:
- Beneath the scab lesions, the potato flesh may remain healthy, or it may exhibit discoloration, such as dark spots or streaks.
- Severity Varies:
- The severity of potato scab symptoms can vary depending on environmental conditions, soil pH, and the presence of specific strains of the causative bacteria or fungus.
Recommendations for Management:
- Crop Rotation:
- Practice crop rotation to reduce the buildup of scab-causing organisms in the soil.
- Avoid planting potatoes in fields where scab has been a recurring problem in previous seasons.
- Soil pH Management:
- Maintain optimal soil pH levels for potato production (around 5.2 to 6.0) as excessively acidic or alkaline soils can exacerbate scab development.
- Conduct soil tests regularly and amend soil pH as needed using lime or sulfur.
- Irrigation Management:
- Avoid over-irrigation and waterlogging, as excessive soil moisture can promote the development and spread of potato scab.
- Use irrigation practices that promote even soil moisture levels without creating waterlogged conditions.
- Resistant Varieties:
- Plant potato varieties that have been bred for resistance or tolerance to scab, if available.
- While no potato variety is completely immune to scab, some varieties exhibit reduced susceptibility to the disease.
Causes:
- Bacterial and Fungal Pathogens:
- Potato scab can be caused by several species of soil-inhabiting bacteria, primarily Streptomyces spp., which thrive in alkaline soils.
- In some cases, the fungus Spongospora subterranea, which also causes potato powdery scab, can contribute to scab lesions on tubers.
- Soil Contamination:
- Scab-causing bacteria and fungi survive in the soil and can persist for several years, especially in fields with a history of scab outbreaks.
- Soil contamination can occur through infected seed potatoes, irrigation water, or contaminated soil amendments.
Preventive Measures:
- Seed Potato Selection:
- Use certified disease-free seed potatoes obtained from reputable sources to reduce the risk of introducing scab-causing organisms into the field.
- Inspect seed potatoes carefully for signs of scab lesions before planting.
- Soil Sterilization:
- In situations where scab is a significant problem, soil sterilization techniques such as solarization or steam sterilization may be used to reduce bacterial and fungal populations in the soil.
- Sanitation:
- Practice good sanitation by removing and destroying scab-infected tubers and crop residues to reduce the source of inoculum in the field.
- Avoid using contaminated soil or compost in potato production areas.
- Chemical Control:
- In some cases, soil drenches or seed treatments with appropriate fungicides or bactericides may help reduce the severity of scab, especially in high-risk situations.
By implementing these preventive measures and integrating them into potato production practices, growers can effectively manage potato scab and minimize its impact on yield and tuber quality. Regular monitoring for symptoms and prompt action are essential for successful scab management.