Five trees of the Hevea brasiliensis clone RRIM 600 tapped 1/2S d/2 on panel BO-1 were affected by bark dryness. It occupied almost all of the tapping cut 15 days after the appearance of bark dryness. Field observations were made on the trees, using bark puncturing and cutting, after 5-30 days of dryness and further observations were made after 12-15 months. In coordination with the field observations, samples were collected for light- and electron-microscopic investigation. Almost immediately after all of the tapping cut had dried up, the whole tapping panel and half of the rootstock under the panel manifested the symptoms of bark dryness with tissue necrosis, and the closer to the rootstock was the bark then the higher the degree of symptoms shown. Bark dryness was also found in one or more lateral roots of each tree and the higher the degree of symptoms shown. Bark dryness was also found in one or more lateral roots of each tree and the dryness was always traced downward to a wound with little cover. It is suggested that the primary lesion of the bark dryness might originate from the root wound from whence the lesion spread to the tapping cut. A model is proposed to explain the origins and spreading of rapidly-spread Tapping Panel Dryness.