Please have a look at this useful link.

https://www.researchgate.net/post/At_which_stage_of_plant_growth_is_boron_application_most_effective_in_a_fibre_crop

 thank you sir Arvind Singh

Kindly see here too.

http://www.cropnutrition.com/importance-of-boron-in-plant-growth

Boron is considered an immobile micronutrient in most of the crops.  If we are working on perennial crops  , best course of boron application would be to use multiple sprays till the newly emerged leaves attain a growth stage of source after initially acting as sink. Enclosed below an interesting PDF on boron nutrition...for further reading..

The sprays of boron depends upon the objective of the study. If it is to be used for enhancing fruit setting, then it should be sprayed during flowering. while if fruit cracking is the problem then boron applications are made during fruit development period.

Interesting  discussion . Most of the times , fruit cracking is associated with sub-optimum boron nutrition . On the contrary , there could be some co-factors as well .  While establishing the fact about the role of boron nutrition in fruit cracking , very often that  right stage of  boron application , we miss it , so while addressing such issues, we also have to keep in mind , whether or not , that spray of boron will yield any anticipated results..? Best course would be to ensure prophylactic spray of boron in those  orchards having  chronic history of fruit cracking , or else established through leaf analysis for boron deficiency  to be established precisely...