 
Black or mushy stems on Euphorbias can be a sign of rot, which is usually caused by overwatering. When the roots are sitting in water for prolonged periods, they can start to rot, which can spread to the stem. If left untreated, the entire plant can die. It is important to check the soil moisture regularly and make sure the pot has good drainage. If the soil is too wet, let it dry out before watering again.
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