Why are there holes or sunken lesions on my potatoes?  Well, there are a few possible explanations, depending on the circumstances.  Are the holes on the surface of the tubers or in the center?  Are they wide or narrow?  Either insects or environmental factors are the likely causes. 

White grub damage on sweet potato (L) and a white potato cultivar (R).

If there are shallow or deep holes on the surface of the potato that are not associated with decay, it may be the result of feeding by white grubs.  White grubs are the larvae of several different beetles and are most widely known for the damage they do to turfgrass by feeding on the roots.   Damage to vegetable crops and herbaceous ornamentals most often occurs in new gardens that were previously grassy areas, or gardens that have grassy weeds.   Sufficient control is usually obtained by managing grassy weeds, or by rotating crops with legumes (beans, peas).   Tilling the soil may reduce populations by killing the larvae either by physical injury or by exposing them to predators and the elements.  This is best done from May through June for older larvae and from late July through September for the younger larvae.   Avoid injuring the roots and developing tubers of the plants!  White grubs were identified as the cause of these holes on potato  and sunken, chewed out lesions on sweet potato.   White grubs may also cause damage to other root crops including beet, carrot, turnip, and rutabaga.   They will feed on the roots of other vegetables and ornamental annuals and perennials.   Strawberry roots are also attractive to white grubs.   White grub damage on vegetables is discussed in more detail in this fact sheet. 

Another insect pest that will chew holes in root crops such as potato and carrot is the wireworm.  Wireworms are the larval stage of click beetles, so named because of the ‘clicking’ sound they make when they right themselves after accidentally winding up on their backs.  These pests have life cycles that vary from 2-6 years depending on the species, so multiple stages of the insect can be present at any given time.  Like white grubs, wireworms are often more of a problem in areas that were previously grassy or in fields or gardens with grassy weeds.   Many of the adult click beetles prefer to lay eggs near the roots of grasses so they are often associated with weedy fields.   When wireworms attack root crops at an early stage, deformity may result.  If feeding occurs later on more developed tubers (or carrots, etc), damage will be in the form of holes or tunnels.   You can check for wireworms by digging holes the size of a softball and filling them with chunks of carrot or potato, covering, and checking in 4-5 days to see if they have been attracted to the bait.   Complete control of this pest where it is established is very difficult.   When numbers are high, insecticides are used.  Another option for partial control is the use of biocontrol agents such as beneficial nematodes.  Check out this fact sheet for more on wireworms.

www.uiweb.uidaho.edu

Cavities in the center of potato tubers that appear healthy on the outside are caused by a condition known as ‘hollow heart’.  The cavities are irregular or angular in shape and have brown edges.  This is associated with rapidly growing tubers.  This is most severe when conditions favor rapid tuber development, especially following drought stress.  Hot dry weather followed by cooler weather with rain may result in these cavities.  Practices that inhibit rapid spurts of tuber growth will reduce the incidence of hollow heart.  This can also be worse where there is a marginal potassium (K) deficiency in the soil.

Hollow heartwww.gov.mb.ca

Hollow heart of potato can be reduced in several ways.  Some cultivars are more prone to this condition, so trying other cultivars is one option.  Use closer plant spacing to increase competition between plants which will reduce the likelihood of rapid growth spurts in the potatoes.  Hill the potatoes.  Maintain uniform and consistent moisture levels in the soil using irrigation during dry periods if possible.  Have the soil tested and add potassium and other nutrients as recommended.  Click here for more info on hollow heart.

 J. Allen