Large growth cracks are due to desynchronized growth between inner and outer tuber tissue. Sometimes this is referred to as bursting. Cracked tubers are culled unless the cracks are shallow enough that the tubers could be used for processing.
Development and Appearance
Longitudinal fissures develop when the core tissue inside the tuber grows faster than the outer tissues, the periderm. In other words, the internal pressure of the tuber is greater than the tensile strength of the skin or surface tissue. The resulting fissure or crack can extend the entire length of the tuber and may be shallow or a half inch deep. Growth cracks undergo wound-healing and show a characteristic suberized appearance. When healed, they seldom become infected with a pathogen. Growth cracking can also be associated with plant infection by a few relatively uncommon viruses such as yellow dwarf virus and spindle tuber viroid. More importantly, growth cracking can be caused by exposure to members of two new herbicide families, imidazolinones such as Pursuit and sulfonylureas such as Ally and Accent.
The sudden and rapid growth of internal tuber tissue or growth cracking is primarily due to an uneven availability of soil moisture and rapid, uneven uptake of water. This occurs when heavy rain or excessive short-term irrigation is followed by a period of dryness. Growth cracking is exacerbated when plants are spaced widely apart or when tuber set is unusually low. Uneven fertilization placement worsens the cracking. This is also true of excessive nitrogen or poor timing of nitrogen fertilization especially during the mid-bulking period. Low soil boron may also worsen the amount of cracking.
Varietal susceptibility to growth cracking is not well documented or understood. It may be related to the speed of tuber growth during mid-bulking and to root growth.
Three cultural practices to minimize growth cracking are uniform plant spacing, adequate soil moisture with consistent irrigation scheduling and uniform fertilization especially avoiding excessive and late applications of nitrogen.
Thumbnail cracking is shallow and random. Appearance is that of semi-circular breaks, half-moons, and are associated with exposure of very turgid (hydrated) tubers to very dry conditions. Surface splitting or air cracking is a slight separation of the skin. This is associated with exposure of very turgid tubers to sudden cold temperatures. Both these types of cracking are associated with harvest but should not be confused with shatter bruise which has a distinct appearance and is caused by impacts. Thumbnail and air cracks heal slowly and are subject to infection and dehydration in storage.
This disorder is called elephant or alligator hide on russet varieties and fish skin or turtle back on red and white varieties. It is characterized by a thick russet appearance to the skin due to furrowing or cracking of the skin. Causes and practices to control it are not known. Russet Burbank seems to be especially susceptible.