Answer: It's so sad when such a spectacular plant starts to fail! The stem rot is likely caused by a fungus called Pythium ultimum. You can prevent the problem by planting tubers in well-drained soil in an area that gets good air circulation. Mulch can hold excess moisture up against stems, giving the fungus the advantage. If you can take a stem tip cutting well away from the rotted tissue, you can try to root it. Otherwise, the plant is best removed and destroyed (sad to say!). If the soil is a heavy clay or has poor drainage, you can build up a raised bed area to plant in, or incorporate vermiculite, perlite and peat moss to improve the soil. Best of luck with future begonias!
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