Hi- unfold the affected leaves to see if you can see aphids feeding. Unless you have a severe infestation affecting a good portion of the tree you might consider thoroughly spraying upper and lower leaf surfaces with insecticidal soap or horticultural oil. In most cases, aphid populations are kept in check by a host of natural enemies- predators and parasitoids.

If you don't see signs of aphids or other insect pests, the curling may have been caused by some combination of environmental stressors (e.g. cold temps as leaves were emerging).

Apples can be susceptible to several insect and disease problems. However, a large number of problems are caused by cultural and environmental factors (nonliving factors). These include too much or too little water or nutrients, insufficient space, sunlight, or support, compacted soil, temperature extremes, and root damage from cultivation. In addition severity of some common problems is closely related to weather (some diseases are worse in wet years) and to gardening decisions (choosing inappropriate varieties, purchasing low-quality plants, incorrect spacing, etc.).

HGIC's apple information, including insect pests and diseases:

https://extension.umd.edu/programs/environment-natural-resources/program-areas/home-and-garden-information-center/food-gardening/fruits-home-garden

Here is an excellent home fruit pest management guide, including spray schedules, from Va Tech Extension that we recommend for MD residents (section 3, beginning on page 85):

 https://extension.umd.edu/resource/apple-and-pear-diseases

To grow fruit successfully, you must be able to grow healthy plants and anticipate and prevent problems. Most diseases can only be controlled with scheduled preventative sprays early in the season. 

Jon