|Adapted from an article by Phil Nolte, extension
seed potato specialist, University of Idaho, that
appeared in the September 1997 American Vegetable
With the end of this year's potato growing season in sight, it is time to prepare for crop harvest and storage. The first two to three weeks after potatoes are placed in storage are referred to as the "curing period," and it is during this period that several important potato diseases get their start.
Two common potato diseases, Fusarium dry rot and Pythium leak, are caused by fungi that need a wound to enter before they can infect potatoes. Wounds occurring during harvest require both time and energy to heal. In fact, a series of events must take place before a wound is no longer vulnerable to invasion by tuber pathogens. The term "suberization" is often used in reference to wound healing, but it is just one of the early events in the potato tuber wound healing process which actually consists of several different steps.
When a tuber is wounded, healing begins in the undamaged cells just beneath the wounded area. First phenolic compounds are deposited into the walls of the outer two or three layers of intact cells beneath the wound. One of these phenolic substances is called "suberin" and is the origin of the term "suberization." Suberin is a complex fat-based phenolic material. The complete structure of it is as of yet unknown, but it looks much like common bottle cork. This suberin layer seals off the wound to prevent the loss of moisture, thus providing protection from bacterial pathogens, such as bacterial soft rot.
The final stage of wound healing involves the formation of a new wound cork or a "phellum" layer. This process occurs within the cells just beneath the new suberin layer, where a series of new cross walls are laid down parallel to the wounded surface. The area where this occurs is a new meristem and is called the "phellogen." After a series of cell divisions, the end result is a layer of flattened, brick-shaped cells usually four to six cells deep. When the cell division phase ends, these wound cork cells also become suberized. The temporary suberin layer collapses because its cells are cut off from the moisture supply within the tuber. This new wound barrier created by the tuber is very similar to the original skin or periderm of the potato tuber both in appearance and ability to protect the healed area.
Under ideal conditions, the wound healing process takes about a week. Suberization usually takes two to four days, and another two to four days for wound cork formation occur. Wound healing also requires temperatures from 50 0 F to 60 0 F, oxygen, and high relative humidity. Wound healing rates are actually quite high at temperatures of 70 0 F or higher, but the activity of most tuber pathogens is so accelerated at these temperatures that the wound healing system cannot react rapidly enough to prevent infection.
Wound healing must also take place after we have subjected potato tubers to seed cutting, which may be the most serious damage that they are ever likely to suffer. Because of the time required for healing, we often recommend seed piece treatment fungicides to protect cut seed until the wound barriers can be established.
As you can see, wound healing in the potato tuber is a complex process. Many tuber pathogens have sufficient time to get established before the wounded tuber is protected. This is one more reason to manage harvesting and handling equipment in a manner that keeps tuber wounding to an absolute minimum. If you harvest tubers carefully and store them in the right conditions, many diseases can be avoided.