How Do You Treat Anthracnose on a Japanese Maple Tree?
Anthracnose is a fungal infection that is most likely to break out in wet conditions. It is rarely deadly but destroys the tree’s aesthetic quality by killing the leaves.
The cause of the infection is one of several types of fungus, with the most notable being Discula campestris, Aureobasidium apocryptum (syn. Kabatiella apocrypta), and Colletotrichum gleosporoides.
This fungal infection will cause defoliation of your Japanese Maples. Healthy trees can withstand defoliation and grow new foliage and recover. Young trees are more susceptible to long-lasting damage, while older, more established trees suffer minor growth losses.
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Symptoms
The most prevalent symptom is the appearance of light brown, papery lesions that develop along the primary veins and margins of the leaves.
These lesions spread until the entire leaf is covered, at which time the leaf is dead and will fall off.
In severe infestations, young twigs and buds can also be affected to the extent that they are damaged and die.
Though Anthracnose does not kill the tree, it does weaken the plant and make it susceptible to other diseases and attacks by insects that may destroy it.
Life Cycle
The fungus produces conidia (asexual spores) contained within acervuli (tissue lumps). These appear as black or dark-brown spots on the upper and lower surfaces of the leaves.
Infected leaves die and drop to the ground, where the fungus overwinters in the dead leaves. It can also overwinter in infected buds and twigs.
In spring, with milder, wetter, and warmer weather, the fungus produces fruiting bodies, and spores are released. The spores are spread by wind or by water splashing during rain.
Treatment
Infected leaves and twigs that harbor the disease should be ruthlessly pruned out as they are the source of the infection. This will reduce the number of spores in the future.
Next, you should ensure that your bonsai grows with as much vigor as possible, so an enhanced watering, fertilization, and mulching routine should be implemented. This will assist the tree in withstanding the ravages of the fungus.
Maintain sufficient distance between your trees to ensure good airflow around them. This creates an unfavorable environment for the development and spread of the fungus. The airflow will dry out any moist areas.
Observe the tree to prevent secondary attacks from other diseases on the Japanese Maple or insects during its recovery.
Chemical Control
If the infection is severe and you are concerned about permanent damage to the Japanese Maple tree, chemical controls can be implemented. This fungus will be destroyed by regular spraying with a fungicide containing mancozeb with copper hydroxide, propiconazole with mancozeb, or chlorothalonil.
Spray just before the buds burst in spring to protect the new growth. If the wet, warm weather continues, continue spraying at the intervals indicated on the package label. 
To Wrap Up
When found in healthy, mature specimens, Anthracnose will, in all likelihood, not kill the tree but will decimate the foliage. Valuable specimens should be chemically treated as soon as the symptoms appear.
Young trees are at risk as they do not have the resilience your older specimens have. These trees should also be chemically treated.
Maintain healthy spacing with excellent airflow, remove any infected leaves and fallen leaves, and ensure they are burned.
Your bonsai should weather an outbreak of Anthracnose, but doing all you can to destroy the spores will go a long way to ensuring it does not return year after year.
