Association of a xylem-limited bacterium with Sumatra disease of cloves in Indonesia

Sumatra disease is highly destructive in most clove-growing areas of Sumatra and in western parts of Java. Symptoms of leaf-fall and twig die-back, starting in the upper branches, and often associated with wilting, lead to tree death in 6–18 months. Normally, only seedlings and young trees survive. Symptoms are usually more severe and disease spread is more rapid at higher elevations. Patterns of jump-spread suggest that the disease agent is carried via an airborne pathway, possibly by insect vectors. Small, Gram-negative, non-motile, rod-shaped bacteria are consistently associated with the disease. Initial attempts to culture the bacteria in vitro did not succeed. The bacteria are confined within xylem vessels and are found throughout diseased trees. Greyish-brown streaks are present in wood of heavily infected tissues which release a bacterial ooze from cut surfaces; in such tissues many of the vessels are occluded by bacteria embedded in a gelatinous matrix. At the time of first above-ground symptoms, bacteria are already well established in roots and the trunk base, from where they appear to spread upwards in newly formed wood into the crown of the tree. Symptoms are thought to result from root dysfunction and mechanical blockage of the vascular system. All known clove varieties appear equally susceptible.