Certified GKH Gardening Expert
Unfortunately, your photo did not come through.
It does sound like a common infection, Rhizopus. The easiest way to control this is to take away the conditions in which it thrives. It likes warm humid conditions, so removing infected growth and proper pruning to allow airflow is necessary.
Treating with a fungicide will also help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
Here is an article to help you with the care of the tree: https://www.gardeningknowhow.com/edible/fruits/jackfruit-trees/growing-jackfruit-trees.htm