Dont forget this thread, lots of info.
Palm disease information
It's fungal and there's lots of docs there, but the one on petiole blight says this which is likely common for most fungi.
Quote:
It is critical to understand that fungicides do not
cure the damage already present on the palm. Plant
tissue does not “heal” itself. Once the damage
occurs, it will remain for the duration of the life of
that particular leaf. Fungicides are used to prevent
further spread of the disease by protecting a leaf
petiole and rachis that has not yet been infected by the
fungal pathogen.
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Seems systemic fungicide for protection is the answer ... which one is a guess. You need to try, that's all. I try between copper based, sulphur based and perhaps something like ausphos600 etc.