What killed all your grass? Maybe that (drought, herbicide, etc.) is what is responsible for the burnt looking leaves on the Jackfruit. I have never heard of Jackfruit needing fungicide. I would be more inclined to attribute the issue to drought, sunburn, or some other kind of stress. Once again, grafted jacks tend to have a weaker root system (especially since the tap root rairly develops properly in a nursery pot) and would be more sensitive than mango or lychee. Mango, by the way is incredibly drought tollerent for a fruiting tree.