I would dig up the crinum, pot and move it to another part of the yard to see if you can nurse it back to health. It's really unusual for crinums to have any issues and the only bugs I've seen atack them are grasshoppers. Maybe there's some type of grub attacking the roots. I have a whole patch of milk and wine crinums growing in the back yard where they never get any care or water besides rain and they've never had an issue.

Kara

Hi,
Crinums can be susceptible to leaf spot disease caused by a fungus. During times of heavy watering or rain, the fungus can spread from ground to plant when water droplets splash on the plants with fungus spores. The suggestion is to remove infected leaves and suckers around base. I am not sure if there is a spray for this. Thrips also will attack Crinum lilies.

Not sure how long you've had yours but my milk & wines are in a spot that gets barely any sun in the Winter & die back to the ground every year, then rebound & multiply through Spring, Summer & Fall. I've wondered if they would grow the year through if I moved them, but haven't yet done that. I've had them for over 20 years now.

Sorry to all who posted that I didn't respond. A thing came up in my non-gardening life. I did read all your advice.
And I have the black spot on some crinums, but not all. Some seem less susceptible to it.
I've fallen in love with crinums because they are so care-free. A month after my original post, I can report that the one that wasn't doing well is still not doing well. But every other crinum in the yard seems fine and the other two that were hit with whatever hit them are both blooming now.
I'm just going to leave it and see what happens. Crinums seem able to take just about anything, including getting mowed down (a small one my husband didn't know about was mowed down two months ago and has come back bigger, as if it was just waiting to please get mowed down.)

And Carol, be really careful moving a crinum that old! My husband and I moved one that wasn't blooming age yet and we still dug more than a foot out from the stem, trying not to damage the bulb. Turns out the bulb had been growing sideways and we shaved at least a quarter off of it by accident. The rest of the bulb suffered a bit for a year, but has recovered. The plant is huge and finally blooming. But I wonder if we lost a year or more because we cut the bulb so much. They can be really huge bulbs, even when the plant doesn't seem huge yet.