Answer: As with any home remedy, it is always best to test the spray on a few leaves, wait a few days and make sure there is no adverse reaction. Not all products whether home made or commercial can be tolerated by all plants under all application conditions.
The foliage was apparently damaged and will fall off, the tree will have to releaf. This is quite stressful on the tree and it may or may not be able to do it. Make sure to water the tree if there is no rain.
You might also want to check with your county extension as to what they are currently recommending for Japanese beetle control on plum trees. Always be sure to read and follow all of the label instructions carefully.
Q&A Library Searching Tips