It depends a lot on what type of galls you found. If they're bacterial, then you'd need a bactericide specific to that bacterium, phyton should work. If it's a mite or larvae, then you'd need an insecticide and bactericides wouldn't do much good.
If it's bacterial, it might be good to find the source as well; agrobacterium tumefasciens can't travel a lot on their own, they need both a host, as well as a vector (a carrier) to spread it to your crab apple. Aphids and beetles are part of those carrier groups. If there are numerous galls in the soil, something must have lived there and infected your tree. But tracing it back to the original source could help you resolve this issue within a year (burn the source and treat your tree), instead of having to combat it every moment from now on.
Please take note that these bacteria will go resistant if not treated until every last one is dead. It happens in every genetic research lab a few times a year.
Treat all of your susceptible trees with the same stuff, both bactericide as well as insecticides, before you have a wildfire of hosts in your backyard.
