Potato Common Scab
Risk near you, next 48h
See whether your local forecast favours this disease right now.
When it strikes
Watch it before you act
At this level the damage rarely costs the crop. Note the date, check twice a week, and act only if it reaches new growth or the weather turns favourable for it.
Wind-blown spores
Spores travel on air well beyond the affected bed, so a neighbouring garden can be the source. Spacing and airflow matter more than distance from your own infected plant.
It carries over to next season in soil, debris or seed — which is why rotation matters here, not just treatment.
Hot compost only
A heap that reaches and holds 55–65°C throughout will destroy it. A cool corner heap will not, and will return the problem to the beds with the finished compost.
When and where to look
Scout the planting while temperatures hold between 18°C and 30°C, in dry air below 60%.
Rule these out first
Same category, and at least one host crop in common — so they are genuinely confusable in the same part of the garden.
The signs that lead here
Full problem lists for these crops
About
Common scab (Streptomyces scabies) is a soil-borne actinobacterium that roughens the skin of potatoes without affecting the flesh underneath. It is a cosmetic problem rather than a crop loss, but a severe attack makes the tubers hard to peel and shortens their storage life.
Symptoms
Raised, corky, brown patches on the tuber skin, sometimes sunken and star-shaped in severe cases. The haulm looks completely healthy — the first sign is at lifting. Beetroot, radish and parsnip growing in the same soil can show the same rough patches.
Organic Treatment
Nothing treats an affected crop, so manage the soil for the next one. Keep the soil consistently moist during the four to six weeks after tuber initiation — that single measure suppresses the bacterium more effectively than anything else available.
Chemical Treatment
No fungicide controls it: the organism is a bacterium living in soil, not a leaf fungus. Sulphur applied to acidify the soil towards pH 5.2 is the only chemical intervention with any evidence behind it.
Prevention
Never lime the potato bed and avoid fresh manure and wood ash, all of which push pH up. Grow resistant varieties, rotate on a four-year cycle, and irrigate reliably while the tubers are forming.
Crops at risk
Related problems
Potato Common Scab risk by location
Each location has a month-by-month profile computed from the weather archive: how many hours a year actually meet this disease’s thresholds.