In Colorado, English Hawthorn (Crataegus laevigata) and its varieties, especially Paul's Scarlet, are very susceptible to leaf spot/leaf blight diseases. After wet warm springs, leaves may turn yellow and drop by August. Small black dots may be spores of these fungal diseases. There are fungicides that can prevent this disease but do not cure it. They would need to be applied to leaves early in the season, well in advance of any symptoms. For now, rake up and discard any fallen leaves through winter. Avoid wetting leaves with sprinkler irrigation. Ensure that the hawthorn has good air movement through it (fewer plants around it to block winds).
Contact your local Extension office and inquire about their plant diagnostic services and specific fungicides for the control of this disease.
