What happened to the leaves of Osmunda japonica?

If there are no pests, it means that the water is not well watered. You may have watered a lot. However, there is much water leakage, and the water retention capacity of basin soil is not good. You should check whether the basin soil is leaking or unsuitable, and you can solve it by replacing the soil and filling it.

Osmunda likes water. Under drought conditions, its leaves are easy to wither and grow poorly.