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Top dying disease
Top dying disease is a disease that affects ''Heritiera fomes'', a species of mangrove tree known as "sundri", a characteristic tree of the estuarine complex of the Ganges–Brahmaputra Delta in Bangladesh and West Bengal. Although an increase in certain trace elements in the sediment deposited where these trees grow may be a factor in the incidence of the disease, its cause has not been fully established.
==History==
About 63% of the trees growing in the Sundarbans are ''Heritiera fomes''. Sporadic instances of top dying disease had been noticed in these trees earlier in the twentieth century but the disease became more acute and extensive after about 1970.〔 An inventory of trees in the Sundarbans in 1985 recorded 45 million diseased trees with nearly half of these having more than half their crown affected.〔Chaffey, D.R.; Miller, F.R.; Sandom, J.H. (1985). A forest inventory of the Sundarbans, Bangladesh. Land Resources Development Centre.〕 No causal agent has been discovered and the dieback is thought to be the result of stressful conditions, perhaps caused by an increase in the heavy metal concentration of the sediment deposited in the delta.
The maximum level of heavy metal contamination of sediments is found to take place in the late monsoon period and the concentration of these elements in the leaf litter varies with the season and does not necessarily correlate with the amount taken up by the trees. A 2014 study found that the successful establishment of seedlings of ''H. fomes'' and the rate of sapling growth were both adversely affected in areas with high levels of top dying disease, indicating that some environmental factors were likely to be significant. However, although there was some correlation between high levels of trace elements and the incidence of the disease, no particular elements could be demonstrated to cause the condition. The researchers concluded that heavy metal contamination might contribute to top dying disease, but was probably only one of several factors in its incidence.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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