Spots on potatoes are a symptom, not a single disease, and the cause depends mainly on whether they appear on the leaves or on the tubers. Fungi, soil organisms, viruses, insects, air pollution and even rough handling can all leave marks. Looking closely at shape, colour and position is usually enough to narrow it down at home. Most spots are cosmetic or controllable, and the fixes are mostly about how and where you grow rather than what you spray.
Spots on Potato Leaves
The most common cause of dark leaf spots is early blight, a fungal disease. Its spots are dark brown to black with rings inside them, like a target.1Tropical Agrobiodiversity. Integrated Disease Management of Early Blight (Alternaria solani) of Potato It shows up first on the older, lower leaves that are starting to age.2Tropical Agrobiodiversity. Integrated Disease Management of Early Blight (Alternaria solani) of Potato Spotting that climbs upward over weeks fits early blight.
A related fungal disease, brown spot, makes smaller marks, from dots up to about 10 mm (under half an inch) across.3PubMed Central. Alternaria diseases on potato and tomato Its lesions don’t form concentric rings.
Ground-level ozone can damage potato leaves too.4Vegetable and Fruit News. Ozone Damage to Tomatoes and Potatoes It causes tiny light-tan flecks under 1 mm, or small dark stipples about 2 to 4 mm across, plus bleaching or bronzing.5Vegetable and Fruit News. Ozone Damage to Tomatoes and Potatoes Like early blight, it appears first on older leaves, and in potatoes the damage can show only on the underside of the leaf.6Vegetable and Fruit News. Ozone Damage to Tomatoes and Potatoes
To check, turn over a lower leaf and look at both surfaces in good light. Uniform peppering across many leaves at once suggests ozone, especially after hot, sunny, still weather.
Losing leaves reduces the energy the plant sends to its tubers, so yields drop. Early blight on leaves rarely rots the tubers you dig in a garden, though. Ozone damage stops when the weather changes and doesn’t spread from plant to plant.
Not every leaf spot is early blight. Late blight produces pale, water-soaked patches with fuzzy white edges in damp weather, not neat rings. It spreads fast and can collapse plants within days, so it needs quick action. If in doubt, photograph the leaves and ask a local extension service or garden centre.
Scabby and Silvery Spots on the Tuber Skin
Common scab is caused by a soil-dwelling bacterium-like organism, Streptomyces scabies.7Journal of General Plant Pathology. Effects of concanamycins produced by Streptomyces scabies on lesion type of common scab of potato It releases toxins that kill skin cells and form the scab.8Journal of General Plant Pathology. Effects of concanamycins produced by Streptomyces scabies on lesion type of common scab of potato You see rough, corky, raised or pitted patches on the skin. It is a blemish, and peeled potatoes are generally fine to eat.
Powdery scab is caused by a different organism that lives in soil and travels on seed tubers.9Plant Health Progress. Powdery Scab of Potato: A Diagnostic Guide When a spot matures, the skin bursts and releases masses of tiny powdery balls.10Plant Health Progress. Powdery Scab of Potato: A Diagnostic Guide The same organism causes small galls (lumps) on the roots.11American Journal of Potato Research. Field Evaluation of Potato Genotypes for Resistance to Powdery Scab on Tubers and Root Gall Formation Caused by Spongospora subterranea Lumpy roots plus open, powdery pustules point to powdery scab rather than common scab.
Silver scurf is a fungal surface blemish disease of tubers.12Canadian Journal of Plant Pathology. Minireview/MinisynthèseIntegrated management of potato silver scurf ( Helminthosporium solani ) The fungus infects only the skin, entering directly or through the tiny breathing pores on the surface.13Cornell University. Silver Scurf of Potato It needs moisture to start, and can be inside skin cells within about 9 hours of contact.14Cornell University. Silver Scurf of Potato15PubMed. Ultrastructure of the infection process of potato tuber by Helminthosporium solani, causal agent of potato silver scurf
Common scab feels rough and corky and is firmly part of the skin. Powdery scab starts as smooth bumps that open to show brown powder. Silver scurf is flat and shiny, most visible when the potato is wet, and affected skins can shrivel and lose moisture in storage.
All three affect mainly the skin, so peeling usually removes the problem. Cut away any deep pits before cooking. Don’t save blemished tubers as seed potatoes, because they carry the disease into next year’s soil.
At harvest, wash or wipe a few tubers from each plant and compare the marks. Note which bed the affected potatoes came from, since these problems can persist in or on the soil. Keep scabby or silvery tubers apart from clean ones in storage, since silver scurf in particular can spread from tuber to tuber.
Dark Spots Inside the Potato
Blackspot bruise is caused by impact, not disease.16Colorado State University Office of Engagement and Extension. Blackspot Bruise in Potatoes When cells break, natural compounds react with air and form dark pigment, like a cut apple browning.17Colorado State University Office of Engagement and Extension. Blackspot Bruise in Potatoes It forms just under the skin near the ring inside the potato, usually at the stem end.18Colorado State University Office of Engagement and Extension. Blackspot Bruise in Potatoes Grey-blue patches under unbroken skin after digging or dropping a sack are usually bruises.
A local calcium shortage in the tuber can kill cells and cause scattered brown spots or a hollow centre.19Potato Research. Improving Potato Tuber Quality and Production by Targeted Calcium Nutrition: the Discovery of Tuber Roots Leading to a New Concept in Potato Nutrition These are growing problems, not infections, and they don’t spread to other potatoes.
Potato virus Y can cause both leaf symptoms and dead, sunken rings or spots in tubers in many varieties.20American Journal of Potato Research. Foliar and Tuber Symptoms of U.S. Potato Varieties to Multiple Strains and Isolates of Potato virus Y Whether tubers are marked depends on the virus strain and the variety.21American Journal of Potato Research. Foliar and Tuber Symptoms of U.S. Potato Varieties to Multiple Strains and Isolates of Potato virus Y Mottled leaves plus ringed tuber marks point to a virus.
Zebra chip is named for dark and light stripes in the flesh that grow darker when fried.22PubMed. Association of ‘Candidatus Liberibacter solanacearum’ with Zebra Chip Disease of Potato Established by Graft and Psyllid Transmission, Electron Microscopy, and PCR It is caused by a bacterium spread by the potato psyllid, a tiny sap-sucking insect.23PubMed. A comprehensive review of zebra chip disease in potato and its management through breeding for resistance/tolerance to ‘Candidatus Liberibacter solanacearum’ and its insect vector Gardeners outside warm southwestern regions rarely meet it.24PubMed. Association of ‘Candidatus Liberibacter solanacearum’ with Zebra Chip Disease of Potato Established by Graft and Psyllid Transmission, Electron Microscopy, and PCR
To find the cause, slice a tuber lengthwise through the stem end to see where the marks sit. Bruises sit just under the skin and fade toward the centre. Calcium spots are scattered through the flesh or around a cavity. Streaks that darken on frying suggest zebra chip.
Bruised, hollow-heart or brown-spotted flesh can be cut out and the rest eaten. Potatoes marked by a virus or zebra chip taste off and look poor, but they aren’t known to harm people. Never replant virus-affected tubers.
Bruising and calcium problems don’t mean the soil is infected, so they don’t limit where you plant next year. A virus lives in the plant and moves into the tubers: if you find ringed marks in several tubers, treat the whole stock as suspect and buy fresh certified seed.
Treating Spots on Growing Plants
Infected leaves left in the soil carry the fungus into next year.25nwpotatoresearch.com. CROP ROTATIONS – CAN THEY EFFECTIVELY CONTROL POTATO DISEASES? Pick off spotted lower leaves and bin them rather than composting them. Keep foliage dry by watering at the base in the morning.
Do this on a dry day and put the leaves straight into a bag, because handling wet foliage spreads spores. Removing a few leaves is reasonable, but stripping the plant bare costs the tubers energy, so take only the clearly spotted ones. At the end of the season, clear fallen leaves and old haulm from the bed as well, since the fungus overwinters on them.
The early blight fungus has become less sensitive to some common fungicides, the strobilurin group, across US growing areas.26PubMed. First Report of Reduced Sensitivity to a QoI Fungicide in Isolates of Alternaria solani Causing Early Blight of Potato in Canada If you spray, rotate fungicide types and follow label rules. Some soil bacteria, like Bacillus subtilis, make natural antifungal compounds and are sold as biological products.27PubMed. Disease Inhibiting Effect of Strain Bacillus subtilis EG21 and Its Metabolites Against Potato Pathogens Phytophthora infestans and Rhizoctonia solani
Sprays have clear limits. They protect healthy leaves but don’t clear existing spots, and no spray removes scab or scurf already on tubers. Bruising, calcium and virus spots need no fungicide at all.
When plants are badly affected, cut down heavily blighted foliage and wait two weeks before digging so the skins toughen. Dig in dry weather to reduce bruising and storage infection. Set damaged tubers aside to eat first.
Preventing Potato Spot Next Year
Growing other crops between potatoes reduces disease build-up in the soil.28Minnesota Department of Agriculture. Potato Fungicide Best Management Practices to Prevent Drift and Minimize Volatilization Plant potatoes in the same spot no more than once every three years.29Minnesota Department of Agriculture. Potato Fungicide Best Management Practices to Prevent Drift and Minimize Volatilization
A simple rotation divides your beds into at least three groups, such as potatoes, legumes and brassicas. Keep tomatoes out of the potato slot, since they share diseases with potatoes. Note which bed had potatoes each year so you don’t lose track.
Common scab is held back in soils with pH 5.2 or lower.30Cornell University. Potato Scab Losses rise quickly with even small increases above 5.2.31Cornell University. Potato Scab Avoid liming potato beds and test the soil before planting.
If the soil is alkaline or well above 5.2, you cannot realistically bring it down quickly, so rotation, clean seed and even moisture become more important. Take care with the other crops in a rotation too. Brassicas often get lime, and lime put on a bed that will grow potatoes the next year can raise scab risk.
Powdery scab and silver scurf travel on seed tubers.32Plant Health Progress. Powdery Scab of Potato: A Diagnostic Guide33Cornell University. Silver Scurf of Potato Buy certified seed potatoes rather than replanting your own blemished ones. Certified seed is inspected for disease, so look for the tag. Inspect the tubers when they arrive and reject any with soft patches, deep pits or silvery, shrivelled skin, even if they are certified. Supermarket potatoes make poor seed: they are not grown or checked for planting and may carry disease.
Varieties differ in how badly scab, blight and virus affect them, so check catalogue disease ratings before buying. Resistance lowers the need for rotation but doesn’t remove it.
Water matters for scab. Keep soil evenly moist while tubers are forming to discourage common scab, and mulch to hold steady moisture. Avoid waterlogging, which favours powdery scab. Even watering and adequate calcium also help prevent internal brown spot.34Potato Research. Improving Potato Tuber Quality and Production by Targeted Calcium Nutrition: the Discovery of Tuber Roots Leading to a New Concept in Potato Nutrition
In zebra chip areas, insect control limits psyllids.35PubMed. A comprehensive review of zebra chip disease in potato and its management through breeding for resistance/tolerance to ‘Candidatus Liberibacter solanacearum’ and its insect vector Everywhere, handle tubers gently to avoid blackspot.
Storing Potatoes So Spots Don’t Spread
Silver scurf thrives on moisture and spreads in storage.36Cornell University. Silver Scurf of Potato37PubMed. Ultrastructure of the infection process of potato tuber by Helminthosporium solani, causal agent of potato silver scurf So let potatoes dry and cure for about two weeks before storing. Lay them unwashed in a single layer in a dry, shaded, airy place, avoiding light and rain. Brush off dry soil by hand rather than washing, since a damp tuber is the setting silver scurf needs.
Store them cool, dark and ventilated, around 7–10°C (45–50°F), in paper or hessian sacks. Avoid plastic bags, which trap moisture.
Sort the harvest first. Set aside bruised, cut or scabby tubers to use first, and check stores monthly, removing any that turn soft or shrivel.
Colder is not better. Fridge-cold storage turns starch to sugar, making potatoes sweet, and the sugary potatoes darken more when fried. A cool shed or garage suits better than a fridge.
Green patches are a different matter from spots. Green skin comes from light exposure, not disease. Green parts contain bitter natural toxins and should be cut away, and dark storage prevents the problem.