Can anyone please help identify what is infesting my mango tree

Hello,

I had a very healthy and fruitful mango tree in my garden and was producing a lot of mangoes up until 2 years ago. But since last year and again this year majority of the mangoes fall very early. And when cut open, the seed or part of the seed has gone black.

Can some one please identify what is this and how I can get my tree back to 100% again. Many thanks.

Here are the images: