The U.S. potato industry estimates losses during storage at right around 7%. Much of this is due to lost water, which is one of the reasons that high relative humidity (RH) is recommended for storing potatoes.
Most weight loss also occurs during the first two weeks in storage, before the tuber skin has completely matured. Unfortunately, little to none of this moisture loss can ever be recovered, but there are some things you can do to minimize it.
Many storage problems are directly related to the somewhat violent act of harvesting. During this operation, tubers are subjected to conditions that cause nicks, cuts, scrapes, and bruises. Management tools include proper harvester adjustment, recommended moisture content in the soil, and careful attention to drops and other potentially severe impact areas on the harvester and handling equipment.
Don’t Harvest In The Heat
One of the most frequent mistakes growers make is to harvest potatoes when conditions are too warm. Quite simply, tuber pulp temperatures above 65°F are to be avoided. This is because every single one of the potato storage diseases mentioned below become much harder to control when tuber temperatures are too high. Avoiding these conditions might require shutting down your operation during times when pulp temperatures exceed this upper limit.
On the other end of the temperature scale, temps below 42°F also are to be avoided. The physical properties of the potato tuber are such that a severe form of damage called “shatter bruising” occurs very readily when tubers are too cold. The large, complex wounds characteristic of this type of bruising heal slowly and erratically, and are an easy entry point for diseases, such as Fusarium dry rot, to enter the tuber.
After the potatoes have been gently placed in storage, suberization and wound healing are key tools in the battle to prevent storage losses, both from outright moisture loss and losses due to disease. Wound healing requires three conditions: Temperatures 50°F to 55°F, high air volumes (oxygen is required), and very high relative humidity (95%). Two to three weeks are required for wounds inflicted during harvest to completely heal. After the healing period, you can reduce temperatures to the range required to meet your desired market.
Besides water loss, other major contributing factors to storage losses are tuber diseases. While late blight, pink rot, early blight, and Fusarium dry rot all can be responsible for loss, perhaps your No. 1 enemy in storage is bacterial soft rot.
Caused by the Pectobacterium species, the disease is quite capable of infecting potatoes in the field under extremely wet conditions. But, it does the majority of its damage as a secondary decay organism in storage by invading tubers through lesions caused by any and all of these other diseases.
Soft rot is perfectly suited for the role of secondary invader because it is able to thrive with or without oxygen. Storage problems can readily snowball because soft rot in tubers tends to generate wet and slimy conditions that are perfect for further disease development.
Soft rot metabolic activity also generates heat, another factor that can exacerbate the problem. Under these warm, wet, slimy anaerobic conditions, the soft rot bacterium can even invade intact tubers through the lenticels. Disease progress can be alarmingly quick once the bacterium gets a toehold. Condensation and dripping within the storage can readily create the perfect environment for soft rot and are to be avoided.
I can’t take credit for this phrase, but it pretty well sums up the challenge of storage management: “A potato storage is a hotel, not a hospital.” In other words, you can’t improve the condition of your potatoes during storage. However, with good management, you can keep them from getting any worse.