Potato scab

What you are looking at

Raised corky patches on the skin of a lifted potato, brown and rough, with ragged edges that often make a rough star shape. On a badly affected tuber the scabs sink into shallow pits, and even then they go only a few millimetres deep.

Scrape one off with a thumbnail. The flesh underneath is white, firm and perfectly sound.

The crop is otherwise normal. The plants grew well, the yield is what it should be, and the potatoes keep and cook as usual once peeled.

Beetroot, radish, swede and carrots grown in the same bed can show the same thing, since the organism is not fussy about which root it roughens.

A watering problem in a disguise

The organism responsible is a soil-dwelling actinomycete, a bacterium that grows in threads like a fungus, and it is a normal resident of almost every garden soil in the country. Its presence is not the variable.

What decides whether you get scab is moisture during one specific window. A tuber can only be infected while its skin is still forming, which is roughly the four to six weeks after the tubers reach marble size, around the time the plants flower. Skin that has set is not penetrable.

If the soil dries out during those weeks, the organism gets in and the crop scabs. If the soil stays moist, it does not.

That is why the same cultivar on the same plot is clean one year and scabbed the next, and why the answer is a watering can rather than a treatment. It is also why scab is worst on light sandy soils, which dry out fastest, and in the sort of dry British June that people otherwise enjoy.

Soil pH is the second lever. The organism does better as soil becomes more alkaline, which is why liming a bed before potatoes reliably produces a scabby crop. Anyone who limed for clubroot needs to keep the potatoes out of that ground.

Powdery scab is the opposite

There is a second scab on potatoes, and its conditions run the other way, so mixing them up leads to precisely the wrong action.

Powdery scab produces raised pimples that burst open to release a brown powder, leaving a rimmed crater rather than a corky patch. It is caused by an unrelated organism, and it is a disease of wet, cold, heavy soil.

Watering hard through tuber formation prevents common scab and encourages powdery scab. On a heavy wet plot with a history of powdery scab, improve the drainage instead and pick a different cultivar.

Common scab on a light dry soil is far and away the one most British gardeners have.

What to do about it

Water thoroughly and consistently from the point the tubers reach marble size, right through the following month. This matters more than everything else here combined.

Mulch the ridges after earthing up to hold that moisture in.

Keep lime off the ground that will grow potatoes.

Use well-rotted compost rather than fresh manure, and dig it in the autumn before.

Choose a resistant cultivar for a light soil.

Build the organic matter up in a sandy plot over several seasons.

Peel the crop you have and eat it, because there is nothing wrong with it.

What it gets mistaken for

Powdery scab makes pimples that burst and release brown powder, and it comes from wet cold soil rather than dry.

Blackleg blackens and softens the stem base and rots the tuber through to the centre, wet and smelling strongly.

Slug damage bores clean round holes into a tuber and hollows out cavities inside it, and the entry holes are smooth-edged.

Wireworm leaves narrow round tunnels a couple of millimetres across driven straight through the flesh.

Skin spot and black scurf both mark the skin as well. Black scurf leaves flat black flecks that look like soil but will not wash off, and neither affects the flesh underneath any more than common scab does.

Apple scab shares the name and nothing else. It is a fungus, on apples and pears.