The appearance of "dry trees" in rubber plantations and the development of Tapping Panel Dryness has been the subject of a considerable number of studies. A distinction can be made between the two main types of TPD through symptomatological and development differences. The total or partial absence of latex flow, or Tapping Panel Dryness in Hevea brasiliensis, may or may not be combined with necrosis. If there is none, it is usually the case of laticifer physiological fatigue induced by over-exploitation of the trees. After a rest period, tapping can be resumed, therefore, this type of TPD is reversible. The other type, necrotic TPD, i.e. brown bast, is a serious histological, cytological and metabolic disruption, which causes considerable disorganization of the entire secondary, including the latex cells. This phenomenon is usually irreversible.