Potatoes are hardy tubers that store well and maintain their nutritive content if handled properly. However, there are a few missteps that can lose a crop for you or can ruin a bag or crate purchased from the grocer or a farmer's market. Leaving potatoes out improperly can cause them to build up toxins, rot or sugar. Storing them correctly can lengthen their storage life.
Potatoes that are exposed to the light, either because they grow with inadequate cover or because they are left exposed after harvest, will turn green. These potatoes begin to develop chlorophyll to utilize the light, as if they were a stem or leaf. This process makes them bitter because they build up an alkaloid called solanine. These chemicals are toxic and can upset digestion or, in large quantities, be dangerous. Green potatoes should not be eaten or left out where domestic or wild animals can eat them.
Potatoes need to be left out properly after harvesting to "harden" or cure. Leave garden potatoes in the ground for one to two weeks after the plants have been cut and discarded to cure, or store them in a dark place at a temperature under 60 degrees Fahrenheit and a humidity over 86 percent. This will toughen the skins and harden over small cuts on the surface for later storage.
Potatoes left in a light, warm, damp place will begin to sprout, as is their natural inclination. Most store-bought potatoes have been given a chemical application to retard sprout development, so commercially purchased potatoes can spend more time exposed than those grown in home gardens. All will eventually try to make new plants.
Potatoes cooled too much will begin to "sugar." The starches will break down into sugars and the taste and nutritional value will be altered. Freezing can destroy potatoes.
Leaving potatoes in an environment that is too warm (over 40 degrees Fahrenheit), leaving them where they get wet or leaving them in the field too long after harvest can allow fungus to develop. Potatoes will rot. Diseases include dry rot, soft rot, leak, late blight and ring rot.