Read This If You Grow Japanese Maple
Now that I have your attention… This post is about a common bacterial disease called Pseudomonas syringae, which frequently affects Japanese maples yet is relatively easy to control. It is often misidentified as Verticillium wilt, as both cause tip dieback. The Pseudomonas bacteria form purply-black stem discolorations, which is the result of a toxin produced by the bacteria which kills cells. Twigs, branches and eventually the entire tree may die. Older and stronger growing trees are less susceptible, as are some varieties. Do not prune maples in the fall (especially in the Northwest) as this disease enters any wound and is encouraged by wet, cool winters. Any cut, at any time of year, should be sealed immediately with a liquid sealant.
Control is three-fold. The first is keeping your Japanese maples strong, healthy, and damage-free. The second and third are related: If you can keep your tree dry during wet and cold periods, that is half the battle. The other is chemical control, which is by copper sulfate or similar bactericide. Consider using Phyton 35, a systemic bactericide. Top spray and bottom drench is recommended.
If you have problems with this disease, try a chemical drench after repotting or after heavy root work, perhaps with ZeroTol. And if you live in an area with wet, cool weather in the fall through spring, seasonal prophylactic spraying is a sharp idea.
Clearing away leaves and especially seeds of maples—like bigleaf or vine maple—is essential after leaf fall as these commonly planted landscape trees are frequent carriers of the disease. Keep leaves and moss away from trunk bases, too.