Answer: If your potato plants appeared healthy all season, then the symptoms you describe can those of bacterial soft rot, caused by a bacteria called Erwinia. It's difficult to tell which of the remaining tubers might be infected because the bacteria can enter through wounds, injuries or even through natural openings (lenticels) in the skin. Check each potato for injuries or wounds and discard those that seem suspect.
Also, potatoes that have suffered from late blight (the infamous Irish Potato Famine blight) often turn to slime, too. If the entire plants blackened and died before their time, blight is probably the culprit. There are several potato varieties that show resistance to late blight; Elba is currently the most resistant potato variety available. Potato varieties with moderate levels of resistance include Kennebec, Sebago, Allegany, and Rosa.
To reduce problems with disease, buy only potatoes that are certified disease-free; don't save potato seed from year to year. Examine your seed potatoes carefully before you plant, and plant only sound, blemish-free tubers. Rotate crops in the potato family (which includes tomatoes, peppers, and eggplant) on a 4-year cycle in your garden plot. Pull up spent plants and clean up garden debris in the fall and do not compost diseased plant material.
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