Probable fungal infection
My Hibiscus has sort of fungal thing, leaf pesticides have not worked and I am about to cut the whole plant down. You are my last resort. Thanks Carlos Quintana
Certified GKH Gardening Expert
That is absolutely a fungal infection. A pesticide will not kill a fungal infection as it is specifically made for bugs.
I would recommend using, both, wettable sulfur and DOLOMITIC lime together (NOT hydrated lime). This will kill off the infection, while repairing the soil from the treatment at the same time. This will allow you to treat the shrub without killing it.
Now would also be a good time to trim it in half. This will force branching and remove some infected material to help treatment along.
This article will help you to care for these shrubs properly: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/how-to-care-for-hibiscus-plants.htm