What’s Wrong With My Potatoes? Tuber Problems by Symptom

Diagnose the Symptom Before Treating

Damage found at harvest does not always come from a disease. Moisture swings, heat, fertility, insects, storage conditions, and pathogens can create overlapping symptoms. Use this guide to narrow the possibilities, then consult your local Extension office or a plant diagnostic laboratory when the cause remains uncertain.

Symptom	Possible causes	First response
Rough, corky surface lesions	Common scab or another skin disorder	Review soil pH, rotation, seed source, and symptom pattern
Small round tunnels or holes	Wireworms or other soil insects	Cut tubers open, inspect damage, and review field history
Cracks, knobs, or irregular shape	Uneven moisture, rapid growth, heat, or physical restriction	Improve watering consistency and soil preparation
Dark cavity in the center	Hollow heart or internal heat-related defect	Review rapid growth, spacing, water, and fertility
Green skin	Light exposure	Improve hilling and dark handling; do not rely on peeling extensively green tubers
Black material stuck to the skin	Black scurf caused by Rhizoctonia may be possible	Distinguish attached sclerotia from soil and seek diagnosis
Soft, wet, or foul-smelling tissue	Bacterial soft rot or secondary decay	Remove affected tubers and improve handling, curing, and storage sanitation
Brown internal netting	Net necrosis or another internal disorder	Do not diagnose from appearance alone; review variety and virus risk

Rough or Corky Skin

Common scab produces corky lesions that may be superficial, raised, or pitted. Risk is influenced by the pathogen, susceptible varieties, soil conditions, and crop history. Avoid raising soil pH without a test, rotate crops when practical, and use certified seed potatoes. Resistance listed for a variety reduces risk; it does not provide immunity.

Cracks, Knobs, and Misshapen Tubers

Irregular watering can cause growth to slow and then accelerate, contributing to cracks or knobs. Compacted or rocky soil can also distort tubers. Maintain even moisture during tuber development and prepare loose soil before planting. Do not assume that every crack is an infectious disease.

Holes and Tunnels

Wireworms and other soil-dwelling pests can bore into tubers. Cut several affected potatoes open and compare the pattern rather than judging one hole. Field history matters because wireworms can persist in areas previously planted to grass or other host crops. Management should be planned before the next planting and tailored to a confirmed pest.

Hollow Heart and Internal Cavities

Hollow heart is a physiological disorder associated with rapid or uneven tuber growth rather than a contagious pathogen. Large tubers may be more affected. Consistent moisture, appropriate spacing, and avoiding excessive fertility can reduce risk, but weather still plays a role.

Greening

Green color indicates light exposure and can accompany increased glycoalkaloids. Prevent it by hilling adequately, keeping harvested potatoes out of light, and storing them in darkness. Discard potatoes that are extensively green or bitter. See Keep the Green Out of the Potato.

Black Scurf and Surface Material

Black scurf can appear as dark, hard structures attached to the skin that do not brush away like soil. Because other marks can look similar, use clean photographs and a diagnostic resource when uncertain. Plant clean certified seed and rotate away from potatoes and related hosts when practical.

Soft Rot and Storage Decay

Soft, leaking, moldy, or foul-smelling potatoes should be removed promptly. Injury, wet handling, poor ventilation, and unsuitable temperature can encourage decay. Cure only sound mature tubers and inspect stored potatoes regularly.

Internal Browning or Netting

Internal discoloration can result from physiological stress, bruising, heat, vascular problems, or disease. Russet Burbank is associated with a specific net-necrosis risk connected with Potato leafroll virus; see the Russet Burbank net necrosis guide. Do not apply that diagnosis automatically to every variety or every brown internal mark.

When to Seek a Laboratory Diagnosis

Seek help when symptoms spread rapidly, recur across seasons, affect a large share of the crop, or may influence what can be planted in the same soil. Preserve representative samples and photographs, record the variety and planting date, and describe watering, weather, soil amendments, and symptom progression.

Related guidance: Use the yellow-leaf diagnostic guide, blight guide, and harvest guide.