Potato Defects

When harvesting potatoes, home gardeners occasionally come across tubers with defects.  Some commonly encountered problems are listed below.

Knobby Potatoes

Knobby or deformed potato tubers develop when tuber growth is halted by drought, high soil temperatures, or some other factor.  When growth resumes, tubers become knobby or misshapen.  Watering on a regular basis (about once a week) during dry periods helps prevent this problem.


Growth cracks typically form when a prolonged dry period is followed by heavy rainfall or irrigation.  The rapid uptake of water by the tubers causes them to split or crack.  Cracking can be reduced by maintaining an even moisture supply during tuber development.  Water on a regular basis during dry periods.

Raised Spots

White, raised spots on potato tubers are due to wet soil conditions.  Potato tubers are enlarged underground stems.  Lenticels are small openings in the tuber surface that allow for gas exchange.  Saturated soils cause the lenticels to swell as gas exchange is impeded.  Affected potatoes may not store as long as normal, but are perfectly safe to eat.

Potato Scab
may cause a corky appearance on the surface of potatoes.
  Photo credit: Purdue University

Rough or Scabby Tubers

Rough, corky patches on the surface of potato tubers is due to potato scab.  Potato scab is caused by the bacterium Streptomyces scabies.  Though unsightly, scabby potato tubers are still edible.  Simply peel the potatoes before use.

The incidence of potato scab can be reduced by selecting and planting certified, disease-free potatoes in spring.  Choose cultivars, such as ‘Superior’ and ‘Red Norland,’ that possess good resistance to potato scab.  Also, rotate the placement of potatoes in the garden.  If possible, plant potatoes in the same area of the garden only once every three or four years. 

Green Potatoes

Potato tubers are enlarged underground stems.  When potato tubers are exposed to light (either in the garden or storage), their skin turns green due to the formation of chlorophyll.  The chlorophyll itself is not a problem.  However, higher levels of glycoalkaloids also develop in the green tissue.  Green tubers have a bitter taste when eaten.  They may also cause an upset stomach and more serious health problems.  Green portions should be cut off and discarded before boiling or baking the potatoes.  

When growing potatoes in the garden, hill soil around the base of the potato plants to prevent the tubers from being exposed to light.  After harvesting, store potatoes in a dark location.

Holes in Tubers

Wireworms (the larvae of several species of click beetles) and white grubs (the larvae of May/June beetles) occasionally feed on potato tubers.  Wireworm feeding results in small, round holes in tubers.  White grub damage appears as shallow, irregular gouges in tubers.  Wireworm and white grub damage most often occurs in gardens that were lawns or pastures in the previous year.  Little can be done to control wireworms and white grubs in home gardens.  Fortunately, these insect pests rarely cause widespread damage to the potato crop.

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