I am not familiar with all the possible problems of such a plant in your area since you are outside the continental USA. That said, I think the symptoms sound like the plant is either suffering from a root problem (soggy wet or dry soil extremes are the most common root problem) or possibly a blight disease of the foliage. Foliage diseases are not common but could happen if conditions are warm and rainy. Sprays of a fungicide after pruning out the brown/black areas would be the best approach to controlling such a problem.
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