Basil loves sun and heat above all else and, the recent heatwave aside, we have had little of either of these things in the New York area.  Your basil is possibly suffering from a fungal disease carried in infected seed stock and spread when rainfall or sprinkling has splashed water from infected soil or neighboring plants.

Removed all infected materials from the plant and dispose of it away from your garden.  Avoid watering basil plants from overhead and when watering, do so in the morning so any leaf wetness will dry during the day.  Make sure that your plants are spaced with generous room for air to flow in the garden. Do not collect seed from damaged plants.

Good luck with your garden.

Courtesy of NYBG Plant Information