it is in danger if left but being a tree fern the trunk isnt realy alive and you can actualy cut it off above the rotting bit and replant it . id imagine its the cold weather got to it . youl probably get other people who know better than me telling you anyway or ask drbob or bamboo as they are very good plants people and im not ok hebburn .

Thanks for your prompt reply,
I was really hoping that the was some type of powder or spray I could use as the patch is about 5" from the crown. But if the only way to save it is cutting it then thats what I'll do. Thanks Noseypotter.

id still ask someone else first and your welcome .

Not sure about cutting that far up the stem, the trunk is basically a mass of roots and i think that close to the crown is likely to affect the growing point. It may just be a fungus thats set in as the trunk is mostly wet, and with cold damp conditions either out or wrapped up, humid and 'warm' its likely to get something. The green showing will be the new growth ready to start growing. Not sure about how to treat, maybe removing as much patch as you can being careful not to remove to much trunk and bin it. Or wash with a fungicide, may help...

Thanks for your reply,
The black patch first appeared August last year. I wrapped it up for the winter. I had a peek at it last week and the patch has grown but as I said there is a green bit. I will follow your advice first as I will try anything before cutting ( the last resort). Thank you for your time. Hebburn