Potato Black Spots Inside: Blackspot Bruise From Impacts
Potato black spots inside a tuber are often a sign of blackspot bruise. Colorado State University Extension says it is impact damage caused by mechanical stress during harvest plus handling.
The short answer
Potato black spots inside a tuber with clean skin usually point to blackspot bruise. CSU Extension says an impact ruptures cells under the skin plus a pigment forms. The spot shows 24 to 48 hours after the knock. Dry tubers bruise more easily. Gentle digging plus handling cut the damage.
What causes blackspot bruise?
CSU says blackspot forms close to the vascular ring, usually at the stem end. After an impact ruptures cell walls, an enzyme oxidizes compounds in the tuber into melanin pigment. CSU says the damaged area takes 24 to 48 hours to change color. The skin stays whole, so the bruise is hard to spot without peeling.
| Option | What it does | Source |
|---|---|---|
| Impact | Ruptures cells | Blackspot bruise in potatoes |
| Pigment | Forms dark spot | Blackspot bruise in potatoes |
| 24 to 48 hours | Color shows | Blackspot bruise in potatoes |
Why do some potatoes bruise more?
CSU says tuber water status sets how likely a bruise is. Dehydrated tubers bruise more easily, since water in the cells cushions the tuber against a knock. Excess or late nitrogen delays vine maturity plus raises bruise risk. Potassium can lower blackspot in fields low in potassium. CSU adds that more dead vines before vine kill can raise bruising from dry tubers.
Dry tubers: more bruising. Late nitrogen: raises risk. Potassium: helps if short.
How do harvest plus storage play a part?
CSU says every stage of digging, storing plus moving tubers should limit bruises. Nebraska Extension notes that tubers taken from storage may show round dark blotches or black spots just beneath the skin. Its guide reviews bruise types plus their causes. Tennessee Extension says to let potatoes dry for a couple weeks before storage. It suggests about 40 F with high humidity and no washing first.
Gentle digging: fewer bruises. Storage spots: under the skin. About 40 F: storage temp.
Troubleshooting
Dark spot only after peeling? CSU says the skin stays intact over a blackspot bruise.
Spots at the stem end? CSU says blackspot often forms there.
Spots after storage? Nebraska says bruises can appear once tubers leave storage.
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Sources
- Blackspot bruise in potatoes, Colorado State University Extension
- Bruises of potato tubers (G2271), Nebraska Extension
- Potatoes (Irish) for the Tennessee vegetable garden (D69), University of Tennessee Extension
FAQ
Why can't I see the bruise on the skin?
CSU Extension says the skin is not damaged, so blackspot is hard to detect without peeling.
How long after an impact does the spot show?
CSU says it takes 24 to 48 hours for the damaged area to change color.
Does nitrogen affect blackspot?
CSU says excess or late nitrogen delays vine maturity plus raises bruise susceptibility.
Does storage cause black spots?
Nebraska Extension says tubers out of storage can show dark blotches or black spots under the skin.
How should potatoes be stored?
Tennessee Extension suggests about 40 F with high humidity. Do not wash them first.
