Answer: How perplexing to have lost so many maple trees! Maples grow best in full sunshine, in well-draining soil. Planting them in raised beds almost guarantees good soil drainage - unless the beds were built over a concrete slab! Maples require about one-inch of moisture per week during the growing season. It's best to apply water slowly so it can trickle down and wet the entire root mass. As for oak root fungus - Japanese maples are generally resistant to the fungus (Armillaria), but many plants are not. The disease remains in the soil because the fungus sustains itself on buried wood (mostly dead roots from a previously infected tree). Again, in a raised bed, this should not affect the roots of your maples. You can have your soil tested to see if there are traces of Armillaria by contacting your local Cooperative Extension office or a private soil testing lab. If your soil is infected, you can plant resistant trees and shrubs.
Maples are susceptible to Verticillium wilt, a soilborne fungus. It attacks trees through the roots and moves into the vascular system. Classic symptoms of Verticillium wilt are that leaves on one side of the tree, or on a single branch, suddenly wilt and die. The dead leaves remain on the branch. If you cut into a branch you can sometimes find greenish streaks or bands that follow the grain of the wood. I'd cut one of the affected branches off and look for streaks in the wood. If in fact the tree has Verticillium wilt, the roots are infected, as is the soil. In this case, I would recommend against planting another maple in the same soil.
Best wishes with your landscape!
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