Hello,

This is one of the rust diseases that affect crabapples. The time to prevent it is in early spring.

It could be cedar apple rust or hawthorne apple rust-. Compare the differences here 

https://www.canr.msu.edu/ipm/diseases/american_hawthorn_rust

 You can compare the details of what you see in the attached article, looking at the backs of the leaves, and any fruit still on the tree. There are four common rust diseases.   Read through the rest of the article for control options-

https://extension.umn.edu/plant-diseases/cedar-apple-rust#symptoms-on-apple%2C-crabapple%2C-hawthorn-and-other-plants-in-the-rosaceae-family-1173260

Raking up fallen leaves and fruit, and pruning out galls on junipers and other plants in your area, are ways to reduce the spores for next year. Keep leaves of the tree as dry as possible, redirecting lawn sprinklers if necessary, and prune the tree if needed to promote good air circulation. 

Apply fungicide sprays as recommended and timed as outlined in the second article if you do not eat the fruit from this tree.. If you have more questions on rust diseases, please ask.