The stem and leaves are turning yellow and I found one black or dark bug. Could it be bugs sucking the nectar out of it or something else?
Asiatic Lily Problems
Certified GKH Gardening Expert
It is possible that the beetle is the source of the problem, but it is just as likely not the source. I would recommend treating the lilies with neem oil. Neem oil will kill both fungus and pest, which either could be your issue. Neem oil is safe for people and beneficial bugs though, so is ideal for treating for several issues at once.