pretty sure its fire blight... at lest thats what i called it all these years ....

https://duckduckgo.com/?q=fire+blight+in+crab+apples&t=ffab&ia=web

its entirely cosmetic...

after buying the house with the tree.. and it spending most summers near totally leafless... i just got rid of the tree ..

there are many resistant varieties available ... and probably better flower performers ... they put the older prone varieties to shame... in re: the flower show..

spraying would most likely be preventative.. meaning.. its too late now ...

ken

It is NOT fireblight! Fireblight looks like the tree has been attacked with a blow torch (not just a few rust spots on the leaves) and is a far more serious affliction than just "entirely cosmetic". It is a bacterial pathogen of major concern in the fruit industry and can be fatal if persistent or not treated.

Regardless, the problem affecting this crabapple is cedar apple rust, a very common fungal disease where both junipers and apples and their close relations coexist. The fungus moves back and forth between the two host species. Unlike fireblight, although mildly disfiguring, cedar apple rust is not a serious ailment and is not fatal.

Cedar apple rust publication

Not fatal, but really ugly when the tree spends most of the season leafless.