Rotting and dropping of guava fruits is due to

Rotting and dropping of guava fruits is due to Helopeltis antonii.
Both adults and nymphs feed on various sites ranging from tender shoots, buds, stems, and even their fruiting bodies to obtain sap. H. antonii possess modified mouthparts that work to form a long straw-like structure known as a "stylet”.
Injured plants are no longer able to allocate their desired resources to fruit/seed production, rather, they are forced to allocate resources and energy to damage control and repair.