Looks like scab. One of the most common problems with crabs, unless scab-resistant variety.

i had one crab with this.. every year.. by about august.. the real heat of summer.. it was leafless ... i wasnt about to start a spray regimen .. and not overly concerned with leaf damage ... none of the other crabs i had.. were affected ....

i finally just got rid of the tree last year .. it was here when i bought the house.. so it was 30 plus years old ... and in decline otherwise ... they are not long lived trees ... relatively speaking ,...

pretty sure... you would have had to strat spraying very early in the season ... as a preventative.. i dont think there is much you can do about it now ... but again.. i wasnt interested in that level of care ... so i will stand corrected on that.. if i am wrong ...

ken

Whether or not you even need to treat is up for some discussion. It depends on how heavy the problem is. Both various apples and crabapples can be prone to scab and in varying degrees. I grew a Gravenstein apple (my favorite variety) at a former garden for many years and it is highly susceptible. But I never, ever sprayed the tree for anything and it still managed to thrive. It would drop some leaves over the course of the season and some fruit would be blemished (but still completely edible), but nothing that got to be unmanageable or that required treatment or even removal.

Good garden sanitation can go a long way to reducing the problem but I'd just monitor for now to see how severe it might become. Doesn't look particularly bad to me. And it is weather-related, encouraged by wet, cool springs.