I must admit that I have not worked with this fungus, however I suspect that the basidiomycete Oncobasidium theobromae since it affects the vascular system can be isolated from the petiole of infected leaves following sterilization and placement on water agar to keep the tissues moist.. Or from the abscission scar as I understand it sometimes forms a plaque of hyphal growth under moist conditions. There is an article in Fungal Biology : Vascular Streak Dieback of cacao in Southeast Asia and Melanesia: in planta detection of the pathogen and a new taxonomy (Samuels et al., 2012) that may be useful. Good luck.

Thank you Dr. Hendricks for your helpful comment and suggestion. The reference will be used. I hope someday I could make a good network with you and make a join research. Have a nice weekend and thanks