Certified GKH Gardening Expert
This is a fungal infection. If none of those treatments are working, then you will find it difficult to find anything that will. It is likely an incurable infection.
The only thing that you can do is provide the perfect conditions to prolong its life. This article will help: https://www.gardeningknowhow.com/edible/fruits/loquat/growing-loquat-fruit.htm