Common scab (Streptomyces scabiei)
Common scab roughens the tuber skin with corky lesions. It rarely reduces yield, but it downgrades fresh-market and seed crops. The bacteria live in the soil, so control comes from variety choice, soil moisture at tuber initiation and rotation, not from a spray.
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| At a glance | |
|---|---|
| Cause | Streptomyces scabiei and related soil bacteria |
| Main damage | Cosmetic: downgrades fresh-market and seed lots |
| Key control | Variety and moisture at tuber initiation |
| Chemical control | No reliable product |
Cause
Streptomyces are filamentous soil bacteria. Several species cause scab, and some tolerate acid soils, so a low soil pH does not guarantee a clean crop.
Symptoms
- Corky, rough lesions on the tuber surface in three common forms: shallow russet-like netting, raised cork-like lesions and pitted lesions. They do not affect eating quality once the skin is peeled.
How it spreads
The bacteria survive in soil and on infected seed tubers. Infection happens in the weeks after tuber initiation, when the young tuber skin is vulnerable, and is favoured by dry soil during that period.
Where the risk is highest
Worldwide. It is a standard rating in European and North American variety lists, and an important quality defect in fresh-market production in all producing regions.
Management: cultural
- Keep soil moist, without waterlogging, through tuber initiation, which in most crops means the first few weeks of tuber set.
- Avoid liming immediately before potatoes where scab is a problem, and use a rotation that does not return potatoes or other scab hosts to the same field too often.
- Plant seed with clean skin, because infected seed carries the bacteria into the new field.
Management: resistant varieties
Variety is the most effective tool. Variety datasheets give a 1 to 9 rating for common scab, and some listed varieties show strong resistance. A Germicopa variety, for instance, is described as having exceptional resistance to netted common scab and fairly high resistance to pitted common scab, and Fontane has limited susceptibility, with ratings in the 5.5 to 7 range (breeder datasheets).
Management: chemical and regulatory notes
No chemical gives reliable control of common scab. Seed treatments may reduce seed-borne carryover but do not clean up infested soil. Always check current national approvals before using any product.
Seed certification
Seed standards limit the amount of scab visible on the tuber surface, and a lot with heavy scab will be downgraded or rejected even though scab is not a regulated disease. Common scab is therefore a practical grading issue more than a quarantine one.
Seed certification schemes set maximum tolerances for disease by class, from Pre-Basic to Certified, and regulated bacteria such as ring rot and brown rot are held at zero in every class (EU Directive 2002/56/EC framework; Scottish SPCS and Dutch NAK schemes as summarised in the Potatopedia corpus).
Sources
- AHDB Potato Variety Database and European Cultivated Potato Database: resistance ratings
- Germicopa and Fontane variety datasheets, as summarised in the Potatopedia corpus
- University extension and peer-reviewed literature on Streptomyces scab management