Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu
Potato harvest is underway in Delaware and each year problems with potato skin set occur in some fields leading to skinning and reduced tuber quality and potential load rejections. The potato tuber is a swollen tip of a stolon, which is an underground stem. The tuber is comprised of an inner storage area, the vascular ring, the cortex, and the periderm or skin. The skin protects the inner parts of the tuber from moisture loss, invasion from disease organisms and other pests, and physical damage. It is analogous to the bark on the outside of a stem.
As the tuber enlarges, the periderm (skin) must continue to develop and cover the outside of the tuber. The skin is therefore in a state of constant growth until tuber bulking is completed. While the periderm is still growing, it is not yet in a fixed state because the underlying cells are still meristematic, that is, they have not matured. This means that the skin can slip when exposed to mechanical forces causing skinning.
The periderm or skin in a potato has three different zones. The critical zone is a meristematic region called the phellogen where skin cells are dividing. To the outside of phellogen are five to six tiers of phellum cells that were produced by the phellogen meristem and are arranged like a stack of blocks. To the inside of phellogen is the phelloderm region which provides the raw materials for the skin growth process.
When the skin on an immature tuber “slips” due to mechanical pressure it is because the walls of the cells in the phellogen meristem are still soft and easily damaged. When the tuber finally stops growing, due to vine death and/or other factors, the “skin set” process is initiated. The meristem stops producing new cells, the cell walls harden off, and everything from the former meristem area to the outside (the entire phellum region) becomes heavily suberized. Cell walls thicken and change chemically to be resistant to shearing. During this process, the periderm becomes tightly bound to the underlying tissues and is then very resistant to mechanical damage and skinning.
The major factors that contribute to final skin set include variety, soil type, cultural and environmental conditions, vine maturity and duration from vine kill to harvest. Varieties differ in their rate of skin set. This is primarily a function of genetics but also late season bulking rates and how the variety responds to the conditions in the field. Early, round, smooth-skinned varieties, such as ‘Red Norland’, do not set a skin as rapidly as a russet type potato.
Since the tuber essentially needs to be no longer bulking or expanding for skin set to begin, you should avoid practices that encourage vine growth late in the season. Nitrogen should not be applied past mid-May with only a few exceptions (excessive leaching rains in sandy soils would be an exception). Vines should be dead for 10 to 21 days prior to harvest depending upon vine maturity or “greenness” at the time of vine kill and variety. Early, red-skinned potato varieties are notorious for having poor skin set, even with adequate time between vine killing and digging. Some early, round, white varieties can also have this problem.
It is common practice to use herbicides, such as diquat, to kill potato vines prior to harvest in Delaware. Vine kill will initiate the skin set process. However, adequate time must be given for this process to be completed. Digging too soon will risk that the potato phellogen cells in the skin have not matured and therefore the tubers will be subject to skinning damage.
We have had instances in Delaware where potatoes that had good skin set after vine kill reversed and had bad skinning losses in digging. Most commonly this has occured where vines were killed for several weeks and potatoes were subsequently exposed to a wet period. The extra soil moisture caused a reactivation of the phellogen cells in the skin. The tubers where exposed to conditions where growth could reoccur and the phellogen cell walls changed chemically to be meristematic again and thus became more subject to shearing damage and skinning.
Some information in this article was taken from “What is skin set and how do we achieve it?” by Phil Nolte and Nora Olsen, Kimberly Research and Education Center, University of Idaho.
