bulb with brown spot

[The Duchess]

Hi, i have a burrageara nelly isler who have a bulb with a brown spot at top. Is a bulb who had a flower spike. At beginning start with leaves, brown tips,i cut them, but still spread , when leaf from top of the bulb become all brown, the bulb start to become brown at top and now is spread to bottom. The bulb have good roots, bark is new, don't stay in sun, i water when bark is close to dry and i don't sprinkle the leaves..the bulb have a new growth, is small 3-4mm but still green. What happens? Is normal or is ill bulb? To cut and to sacrifice the new growth? Please help me!

Plecase excuse my bad english. :-)