Hi @Ravindran,
Welcome to the Bunnings Workshop community and thank you for your question.
Preventing fungus from appearing in the first place is the best protection for your apple tree. This can be done by ensuring it is not overwatered, pruning the tree to allow airflow and promoting overall plant health.
Check out How To Grow And Prune An Apple Tree for a bit of guidance on caring for your apple tree.
If fungus does appear, identifying the type of fungus is important to ensure you treat it appropriately. Applying Mancozeb Plus Garden Fungicide And Miticide or Liquid Copper Fungicide are common approaches, but the specifics of your issue would need to be considered when choosing an approach.
If you have further details or some photos you can share, I'd be happy to assist further. Let me know if you need a hand uploading images, I'm happy to help if required.
Jacob
