Thanks for sending these photos in! 

It would be helpful to have some more information. Grapes are susceptible to many diseases, so pinpointing which one is difficult from photos. Grapes grown commercially are sprayed frequently to  head off disease, but that is less practical in urban settings esp. since any drift could affect urban neighbors. 
One way to get more information is to put a diseased leaf in a plastic bag with a dampened paper towel to see what type & color of spore is produced, and get back to us. 

Conditions are a factor as well, for example, grapes stressed by shade or drought are more apt to develop botrytis rot or downy mildew, meanwhile Black Rot thrives in warm, humid weather & is one of the most common diseases of  grapes in our coastal plain area of MD/DC/VA.

The leaf lesions look brown, which would likely be Black Rot, yellow lesions would likely be Downy Mildew. The plastic bag spore test would help narrow it down.

Meanwhile, in case it is Black Rot, there are some cultural practices that both help manage Black Rot and have no downside if the culprit turns out to be something else, or more than one:

1. Remove & discard any dead or sickly plant parts, clear & cultivate the ground below

2. Prune for good air flow

3. Bag the fruit when quite small, pea size.

Are the vines in full sun or have some shade? Is there still air around the vines or does air circulate freely between vines? What variety of grapes are they?

Looking forward to hearing more about your grapes!

Kate

https://plantpathology.ca.uky.edu/files/ppfs-fr-s-24.pdf

And here is the link to UMD link to growing grapes in the home garden!

Fortunately, there are Muscadine grape varieties hardy in DC, such as Ison  & Carlos Bronze Muscadine, which gives us some more disease resistant options. 

 https://extension.umd.edu/resource/growing-grapes-home-garden/

Please keep in touch!
Kate