Thank you for your question and picture to Extension. I have a couple of questions for you before we can determine it is early blight on your potatoes.  Is this the first time you have used your grow bag for potatoes? Reusing grow bags may spread the fungus that causes blight.  Did you purchase blight-resistant red potatoes? This is the safest way to guarantee you are disease-free in your growing season.

From the picture, I can see round brown spots on the foliage. Unfortunately, that is a characteristic sign of early blight. Your leaves also look yellow, so it will help to fertilize one more time. Use a 10-10-10 as they may need some nitrogen. Here is information from Extension:

Growing potatoes in home gardens | UMN Extension

You can remove up to a third of your infected leaves on your plant and not harm it. Be sure to not get the leaves wet as this spreads the fungus and wash your hands to avoid spreading the disease. This publication does not recommend applying a fungicide in a home garden, but has helpful information on dealing with blight:

Early blight in tomato and potato | UMN Extension

Good luck and happy gardening!