Maple trees can have any number of diseases and other problems that can cause leaves to develop spots, turn yellow or brown--or die. Among the usual suspects for dying leaves are several fungal diseases and aphids. One of these fungal diseases, Verticillium wilt, is prevalent among Japanese maples. Verticillium wilt is especially serious; it can kill your tree. Other fungi are less insidious.
If the leaves of your Japanese maple have leaves that are wilting and turning yellow or brown, it might have Verticillium wilt. This wilt is caused by a fungus in the soil. It spreads from the roots upward through the sap to the upper branches, causing large limbs to die. You'll sometimes see olive-colored streaks on the wood under the bark.
The county extension service in your state can examine a piece of the bark to see if it has this fungal wilt. If you remove the diseased branches and keep it well watered, an otherwise healthy tree can sometimes heal itself; however most infected trees die a year or two after showing symptoms. Ordinarily the best thing to do is to remove the tree, to keep the disease from spreading.
If you look closely at the undersides of wilted, brown leaves, you might see small green insects that leave behind a sticky, honey-like dew. These are aphids, and they are eating the leaves of your tree.
There are several ways you can get rid of aphids. You can blast them off by spraying them with water. Kill them with insecticides or introduce a natural predator to your tree. Or spray them with a solution of two teaspoons of laundry detergent to one quart of warm water. (That eliminates their protective coating and causes them to dehydrate.) It's also possible to spray them once a week with a solution of dishwashing soap and horticultural oil.
Aphids like fast growing plants. If you fertilize your Japanese maple, use a slow-release fertilizer.
After an unusually cold, wet winter, your leaves may turn purplish brown along the veins of the leaf. Anthracnose is a group of related fungal diseases that causes the early loss of leaves, but these diseases usually only strike once. To prevent an anthracnose fungus from returning, remove and destroy the twigs and branches that have the infected leaves.
Tar spot starts out as yellow spots that turn into large, black spots on leaves that look like tar. It thrives on fallen leaves during the winter and spreads to new leaves in the spring during most weather. It is an ugly fungal disease, but is mostly harmless.