If you've noticed a musty odor around your houseplant, the cause is most likely from overwatering. When plants are given more water than they need, the result can lead to mold or fungus growth, or even root rot, which can destroy the plant.
"Soil kept too moist becomes sticky and slimy, thus inviting root rots and other disease problems," says University of Missouri extension agent David H. Trinklein. Water your plant only when it needs it; typically, when the top of the soil appears dry. Water thoroughly, then allow soil to dry out before watering again.
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Even if you do not overwater your plant, plants that are growing in poorly-drained soil, or pots with out sufficient drainage can still become too wet. Potting soil breaks down and becomes too spongy to drain — repot your plant according to its specific needs to prevent soil breakdown. Plant in a pot that has drainage holes.
If your plant has a musty odor, withhold water until the soil dries out. If the plant still smells, or has visible fungus or mold on the surface of the soil, repot in fresh soil. When repotting, check for slimy or malodorous roots — this is a sign of root rot, and usually means the plant should be discarded, unless most of the root system is still in good condition.