Potato Scab: Causes and What to Do

Dug up potatoes with rough, corky, scabby patches on the skin? That is almost certainly common scab — and the good news is it is purely cosmetic. It looks unpleasant, but the potatoes underneath are perfectly fine to eat. Peel them and you would never know.

Common scab is one of the most frequent surprises for beginners growing potatoes. It is not a sign you did anything badly wrong, and it does not spread to make the crop inedible. Here is how to spot it, why it happens, and how to get smoother skins next year.

How to recognise it

Common scab shows up as raised, rough, brown or tan patches on the skin of the tuber. The patches feel corky and dry, sometimes with slightly ragged edges, and they can be scattered or clustered.

It only affects the skin. Cut a scabby potato open and the flesh inside is clean, white and normal. There is no rot, no smell and no slime — if you find those, you are looking at a different problem.

Why it happens

Common scab is caused by a soil-borne organism (Streptomyces) that thrives in particular conditions. It is far more common in some plots than others, and the usual culprits are:

• Dry soil while the tubers are forming. This is the biggest single factor. A dry spell in early to midsummer, just as tubers are swelling, gives scab its window.
• Alkaline, limey soil. Scab loves high pH. Recently limed ground, or naturally chalky soil, makes it worse.
• Light, sandy or stony soil low in organic matter. These soils drain fast and hold little moisture, which feeds back into the dryness problem.
• A susceptible variety. Some potato varieties scab badly; others shrug it off.

If you garden on free-draining ground and had a dry June, scab is almost expected. It is a soil-and-weather problem far more than a "you did it wrong" problem.

Are they edible?

Yes — completely. Common scab is skin deep and harmless. Once you peel the potatoes, the scab goes with the peelings and you are left with perfectly good spuds. Flavour, texture and safety are all unaffected.

The only real downside is appearance, and a little extra peeling. Badly scabbed tubers store and eat just as well as clean ones, so there is no need to throw anything away. Keep them as you would any potato crop.

How to reduce it

You will not eliminate scab entirely on a prone plot, but you can cut it right down:

• Water consistently as tubers form. This is the most effective fix. Keep the soil reliably moist from when the plants flower through the following few weeks — a good soak in dry spells matters far more than a daily splash. Earthing up helps hold moisture too; see earthing up potatoes.
• Build up organic matter. Working in garden compost or well-rotted manure improves moisture retention and nudges conditions away from scab's favourite dry, alkaline soil. Our guide to improving your soil walks through how.
• Do not lime before potatoes. Lime raises pH and makes scab worse, so skip it on the potato bed. If you are liming for brassicas, do it elsewhere in your crop rotation — well away from where the spuds will go.
• Choose a resistant variety. Some varieties are noticeably scab-tolerant. 'Charlotte', 'Maris Peer', 'Lady Christl' and 'Nadine' tend to come up cleaner on prone soils, so they are worth a try if scab is a regular visitor.

For everything else about growing a good crop — chitting, planting depth, earthing up and harvesting — head back to the main potato growing guide or browse more problem-solving guides.

P.S. If your tubers are turning green rather than scabby, that is a separate (and important) issue — see green potatoes.