Crab apple tree spraying
The only major concern in crabapples is apple scab which can defoliate the trees making them weaker and weaker as time goes on.
The controls are the same as for apple trees a strict fungicide spray program beginning in the early spring as green tissue is exposed. Commercial apple growers will spray every 7 to 10 days with fungicides to protect the leaves and fruit until about a month after bloom when the overwintering inoculum is exhausted. Since you are not harvesting the fruit you don't have to worry about fruit pest destroying your crop so you can skip insecticide sprays.
For home apple grower sprays where you need to protect against insects, and diseases I recommend this short resource from the University of Kentucky https://plantpathology.ca.uky.edu/files/ppfs-fr-t-18.pdf