Answer: The contributing causes include cold night temperatures, cold winds and/or waterlogged soils. Sometimes these stress a plant so much that it develops a disease called Phytophthora, which is a root rot. If the plant were mine, I'd allow it to finish blooming then dig it up, amend the soil so it drains quickly, then replant (as long as the roots appear healthy). This should prevent further symptoms. If you find the roots and/or bulb are mushy or spotted, I'd toss the plant and find a new one. If you replant in the same spot, remove the affected soil and replace with fresh soil prior to planting. Be sure to amend the soil with organic matter to facilitate good drainage.
Best wishes with your beautiful lily!
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