A ripe plum is rich in color with no brown spots or shriveling. When you give the plum a squeeze, it should have a slight give, but no mush. If it's ripe and you are not ready to eat it, pop it in the refrigerator right away to keep it fresh. Leave it out on the counter, and it will go bad quickly.
In the Fridge
Whole plums do not need to be refrigerated, but doing so prolongs shelf life. Stored inside an open plastic bag inside the refrigerator, a whole plum keeps up to 5 days, but on the counter for considerably less time. Sliced plums require refrigeration inside an airtight container or tightly sealed plastic bag within 2 hours, staying fresh in the refrigerator for approximately 2 days.
Plucked early, plums continue to ripen at room temperature. If they're refrigerated too soon, plums lose some of their juicy goodness. Expedite ripening by poking holes in a paper bag, inserting the plums in a single layer with their stem ends down. Close the bag and set it on the counter, checking daily for ripeness. Refrigerate once the plums develop their deep, dark color.
Throw It Away
Like all fresh fruit, plums are perishable. Discard sliced plums not refrigerated within 2 hours to avoid foodborne illness. Toss any plums that contain mold, leak juice, feel slimy or have an unpleasant odor in the trash. If you are unsure whether a plum is spoiled or unsafe to eat, throw it away.