It looks to me like a bacterial rot called black rot. The only treatment is to remove the affected leaf or trim away the affected portion. If it is still in a bucket the addition of a half cup of Hydrogen Peroxide to the water when it is changed or every 3 or 4 days could help. It may be fatal even with treatment.

well thats not good news:( do you know how they get that? or how i can prevent it?

Sorry, it sometimes comes with cold temperatures or a bird or insect brings it in or there is debris in the pond or it was contaminated somehow. Lotus is genetically sensitive to it as are waterlilies. Many people who grow lotus from seed add Hydrogen peroxide to the water they soak the seeds in and continue to add it until the weather turns warm as a preventative. Handling the seed once it gets soft can spread it so wash your hands thoroughly first. There are also fungi and viruses that cause similar problems and a disease called Brown spot.

The big problem with removing the leaf of a lotus is the possibility the plants roots will rot. Some control in research has been partly successful but the use of copper solutions are deadly to pond life. Nothing has been approved for sale that I have heard or read.

I haven't looked at any new info coming out but a quick scan with google didn't reveal anything. If you find something, please let us know.