Hi Fran, thanks for the question. 

This looks like apple scab. A common fungal disease of apples and crabapples.  Our spring weather may have created an environment conducive to this condition.  Make sure you clean up any fallen leaves since they can carry the pathogen.  Pruning your crabapple tree will enhance airflow through the branches reducing the disease risk by helping the leaves to dry faster. Improving overall tree health (e.g. mulching with wood chips, providing adequate water during dry spells, etc.,) will help the tree be more resistant to infection. If needed, apply a fungicide like mancozeb or chlorothalonil, following the product’s instructions carefully. Keeping your tree well-maintained is key to preventing apple scab. Check out the following links for more information:

Apple Scab on Apple and Crabapple Trees | N.C. Cooperative Extension (ncsu.edu)

Is your crabapple losing leaves? Apple scab may be to blame | Good Growing | Illinois Extension | UIUC

Apple scab of apples and crabapples | UMN Extension

I hope this is helpful.