Root diseases are particularly dangerous, because their early stages generally pass unnoticed, and very often the first indication of their presence is given by the death of the tree. Sometimes the main tap-root may be entirely destroyed without any sign of injury appearing above ground, since sufficient supplies of water and salts are provided by the lateral roots to maintain the crown of foliage in a condition of apparent health. One day a storm of wind brings down the whole tree, and the full extent of the injury is disclosed. Occasionally a single lateral root may be discovered showing symptoms of disease. In such a case it may be possible to cut away the diseased portion and to save the remainder of the tree. In the majority of cases, however, the planter must make up his mind to destroy the affected tree in the hope of saving its neighbours.
Altogether, three different root diseases are at present recognised, which may be readily distinguished from one another on examining the affected roots. In the case of the fungus known as Fotnes semitostus, which is the most serious of the three, the dead root is found to be covered with white or yellowish threads and cords of mycelium the growing strands of the fungus. An equally common, but not quite so destructive, fungus is Hymenochaete noxia, which may be recognised from the fact that the root killed by it is covered with an encrustation of sand and small stones, cemented together by the brown or black mycelium. There is a third less common fungus which is occasionally found upon the roots of Hevea in Ceylon, known as Sphaeros-tilbe repens. When this is present there is no external mycelium on the surface of the root, but on peeling off the bark, dark brown strands of mycelium are found wandering over the surface of the wood.