Why Do My Potatoes Have Rough Warty Skin?

Rough, corky, raised patches or sunken pits on potato skin — sometimes just a few spots, sometimes covering the whole surface — are common scab. It is one of the most widespread problems in potato growing and one that frustrates growers because it makes an otherwise perfect potato look unsellable and difficult to peel neatly. The important reassurance is that scab is entirely cosmetic: it affects only the skin and a shallow layer beneath it, and the flesh inside is completely unaffected in taste, texture, or safety. Here is what causes it and how to reduce it.

What causes common scab

Common scab is caused by Streptomyces scabies, a bacterium found naturally in most soils worldwide. It infects developing tubers through the lenticels (natural openings in the skin) and causes the corky, rough tissue that forms the scab lesions. Streptomyces scabies thrives in dry, alkaline soils — it is most active at soil pH above 6.5 and virtually inactive below pH 5.2. Two factors promote scab: dry soil during the period when tubers are forming and sizing (approximately four to six weeks after the plants emerge), and recently limed or alkaline soil. Adding wood ash to a potato bed, for example, reliably makes scab worse.

Watering to prevent scab

The most effective single action to reduce scab is consistent watering during the critical tuber initiation period — from when you first see flower buds appearing until about three weeks later. During this window the tuber skin is soft and vulnerable to Streptomyces infection. Keeping the soil moist (not saturated, but consistently moist) during this period significantly reduces infection. Water restrictions or dry spells during this phase are the most reliable precursor to a scabby harvest. A mulch of compost or straw at this time helps retain soil moisture between waterings.

Soil pH management

If scab is a persistent problem in your garden, test the soil pH. Potatoes prefer a slightly acid soil — pH 5.5–6.0 — and at this level scab is significantly reduced. Do not lime your potato bed. Adding composted pine needles, leaf mould, or sulphur chips can gradually acidify an overly alkaline soil over seasons. Avoid any alkaline amendments in the potato rotation. If you grow brassicas in a neighbouring bed and lime heavily for them, keep the potato ground at the other end of the rotation as far as possible from any lime application.

Variety choice

Some potato varieties have notably better resistance to scab than others. Cara, Wilja, and Maris Peer are among varieties with reasonable scab resistance. Varieties like Desirée and King Edward are significantly more susceptible. If scab is a consistent problem in your soil and garden, switching to a more resistant variety removes much of the problem without requiring any change to cultural practices. Always use certified seed — home-saved seed from scabby potatoes carries the pathogen directly into the new season.