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Common Scab

Common Scab

Streptomyces scabies

Bacterialalso: Potato scab, Scab

A soilborne disease that roughens the skin of potatoes and other root crops with corky, scabby lesions. Caused by a soil bacterium, common scab is mostly cosmetic, the tubers are fine to eat once peeled, but it downgrades the crop and is notoriously hard to control, favored by dry soil and high pH.

🔎 How to spot it

Look on the tuber skin for circular, rough, corky lesions with raised margins, which may be shallow and russet-like, raised and warty, or sunken and pitted into the tuber. The spots can stay small or merge to cover much of the surface. The flesh underneath is usually sound, and plants above ground look normal.

🥀 Damage it causes

Scab is mainly a surface blemish, so the tubers remain edible once peeled, but the corky, pitted skin downgrades the crop, complicates peeling, and can lower storage quality where lesions are deep. It does not kill the plant, but a badly scabbed crop is far less marketable and appealing.

🔬 What causes it

Common scab is caused by the soil bacterium Streptomyces scabies, an actinomycete that lives on organic matter in the soil and rides in on infected seed tubers. It infects young, developing tubers through the skin and natural openings. It is favored by dry soil during tuber formation, warm temperatures, and alkaline soil above about pH 5.5.

🛡️ Prevent it

Plant certified, scab-free seed potatoes and rotate away from potatoes and other root crops. Keep the soil evenly moist during the few weeks of tuber formation, since adequate moisture strongly suppresses scab, and avoid liming the potato patch or raising pH, since scab likes alkaline soil. Grow scab-resistant varieties and avoid fresh manure, which can worsen it.

🧯 If it is already here

There is no in-season cure, so the controls are cultural and aimed at next year: keep tubers evenly watered during their formation, hold the soil pH low, use resistant varieties, and plant clean seed. Scabbed tubers already harvested are perfectly fine to eat once peeled, and there is no need to discard the crop.

💡 Good to know

Common scab is one of the few plant diseases where keeping the soil a bit acidic and avoiding lime actually helps, the opposite of clubroot advice, so do not lime a bed you grow potatoes in. The single most reliable control is keeping the soil evenly moist while the tubers are sizing up. The blemish is skin-deep and harmless to eat.

🌱 Plants it affects

51 plants in the library can be affected by this problem

Adirondack Blue Potato🥔All Blue Potato🥕Atomic Red CarrotBeauregard Sweet Potato🥕Black Spanish Radish🥕Bolero CarrotBosc PearBraeburn AppleBull's Blood BeetChampagne LoquatChantenay CarrotCherry Belle RadishChioggia BeetCortland AppleCosmic Crisp AppleCosmic Purple CarrotCylindra BeetDaikon RadishDanvers CarrotDetroit Dark Red BeetDragon Carrot🌸Easter Egg RadishFingerling Potato🥕French Breakfast RadishFuji AppleGala AppleGerman Butterball PotatoGolden BeetGranny Smith AppleHakurei TurnipHoneycrisp AppleImperator CarrotKennebec PotatoMcIntosh Apple🥕Mokum CarrotNantes CarrotNorland Potato🥕Paris Market CarrotParsnipPecanPink Lady Apple🥔Purple Majesty PotatoPurple Top TurnipRed Delicious AppleRed Pontiac PotatoRusset PotatoRutabagaSuebelle White Sapote🥕Watermelon Radish🥕White Satin CarrotYukon Gold Potato

For educational and informational purposes only. Disease management advice is general guidance drawn from university cooperative extension sources; always identify a problem positively and read and follow the label on any product before use, especially around food crops, children, and pets.