I have been growing a begonia given to me by my Mother for approximately 30 years. I believe it's a type of Rex begonia, but I'm not sure. I have successfully rooted many new plants through the decades, in an attempt to keep the plant going through the years. The main plant is approximately 4-feet across now, lives in a bright sunroom and up until now has been very healthy and trouble free. Recently the tip of the stems on many leaves have developed a brown "stain" where the stem attaches to the leaf...the problem worsens, the leaf and stem begin to wither and eventually fall off. No other part of the plant seems affected. Can anyone help?????

I'm going to try and post a photo of the plant here:

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