Why has my maple got black dieback on its branches?
This is nearly always a bacterial dieback, (it is rarely Verticillium wilt). This condition can first be seen as a dark or black lesion along or around the main branches. As it is caused by a bacterium, treatment by fungicide is useless. The immediate pruning out of the infected area is the only method of control we have found to be effective. Always cut back to just above a pair of buds or side branch down to green, healthy wood. Sometimes this pruning has to be fairly drastic but the good news is that your lovely tree will make a full recovery if this condition is caught and treated early enough.
Dieback can also be caused by soft, unripened growth getting frost damaged during the winter. This growth has been made late the previous season and not had enough time to ripen off before the onset of winter. This is more common in container grown plants that have received too much nutrient at the end of the season. Prevent by pruning out soft, sappy growth in October before it has time to be damaged by the frosts of winter.
Light dieback can often be seen in some varieties of Japanese maple that produce thin, twiggy growth. This is a natural occurrence, which, if left unattended can become unsightly. A few minutes of breaking off this silvery-white dieback will enhance the look of your maple and ensure there is no opportunity for infection to occur. This can be carried out any time but is more easily done in the winter.