How do I manage yellow leaf disease and fruit rot in my arecanut (supari) garden?

You likely have two separate problems. Fruit rot, also called mahali or koleroga, is a fungal disease that flares in the monsoon and rots the nuts, and it is managed mainly by prophylactic Bordeaux mixture spraying on the bunches before and during the rains, repeated as the rain continues, following the recommended schedule. Yellow leaf disease is a more serious phytoplasma related decline that gradually yellows the leaves and reduces yield over years, and it has no simple cure, so management focuses on good nutrition, drainage, removing severely affected palms and using clean planting material. Get the affected palms checked by your KVK or the arecanut research station to confirm which problem you have, and follow their current Bordeaux spray timing and dose for fruit rot, observing label and safety guidance.