Unfortunately, your basil is suffering from downy mildew. This fungal disease has appeared in Maryland in the past couple of years. It blows in on southerly winds. You should salvage as much of the foliage as you can now and make pesto. Ingestion off infected leaves will not hurt you, but you do want to harvest the best leaves.
thank you. is there anything that I can do in the future to prevent this problem?
Plant early and harvest early. This problem seems to crop up in late June, early July.
Downy mildew is a fungal disease and can only be prevented by spraying with a fungicide before the symptoms occur. Basil doesn't give us this option since we want to consume the leaves.