My cane begonias have leaf problems. The tips or sides of a leaf turn brown, curls, and wrinkles. The spot becomes thin and papery. The area enlarges as time goes on. Other leaves become infected.
I see no spider mites, webs, or any other bugs.
The web offered no solutions or photos for me to ID the problem.
What is it? How do I fix it?
You cannot fix the problem once it happens. It sounds like an older leaf that may just need trimming or completely cut off.
It could be a fungal problem so spraying with fungicide may help prevent future problems. If the leaves stay wet too long this could also lead to fungal outbreaks. Any damage to the leaves also are open pathways for pathogens.
Some sun exposure and air movement will help tremendously keep your plant happy and healthy. Regular feeding and spraying will help as well.