Q: Our Japanese maple appears to have some type of fungus. Actually, it looks like small mushrooms. Is this harmful to the tree and what causes this?
A: You have an excellent example of wax scale. The adult females you’ve noticed protect themselves with a waxy outer coating as they suck plant sap. Their red eggs are deposited under the wax coat. If you can physically remove all of the adults, that’s a good first step.
Young wax scale, though, are camouflaged on plant stems. In winter even the old standby horticultural oil will not suffocate many of them because they are not active in cold weather. Even so, a thorough spray with the oil after February on a warm afternoon may kill some percentage of the insects.
The key to scale control is timing your spray to kill the crawlers just after they hatch from the eggs in April. Use carbaryl (Sevin) three times, at two-week intervals, starting in mid-April to control the baby scale as they emerge from under the adults.