Lodewijkp

spray them with a fungicide before storing - it really helps.
you can also pour candlewax in the crown when they are dry - i also do that with robusta's and trachycarpus.
concentrated coconut oil has anti fungal properties.

the best thing you can do like dablo said is cut infected tissue.
or you can cut it back 8 inches above ground, just don't cut to low.
if you wait or you cut not enough it can rot further, so cut at least one nanner deeply and spray it with a mild fungicide or natural fungicide.

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