Basil is susceptible to a few different disease that cause similar looking leaf spots. Your photo most resembles bacterial leaf spot disease or a fungal disease called Downey Mildew. We can’t say for certain its either one of these from a photo; We’d need to examine the underside of the leaves to confirm the mildew. In any case, you’ll want to read more about these diseases in addition to removing the leaves from your plant/garden, reducing humidity and avoiding splashing water on leaves. You may end up removing the entire plant if its bacterial. First, however, we recommend you review the second link included below from VegetableMDOnline/Cornell University because this mildew disease is of major concern in agricultural production of basil in your area of the U.S. Cornell asks that all suspected downey mildew of basil be reported! You’ll want to follow-up right away so you can remove the leaves or plant from your garden, but you’ll be asked for more information about the issue. If it turns out to be bacterial leaf spot, the third link provides more info about that disease.
http://plantclinic.cornell.edu/factsheets/basildiseases.pdf
http://vegetablemdonline.ppath.cornell.edu/NewsArticles/BasilDowny.html
http://uconnladybug.wordpress.com/2011/06/30/black-spots-on-basil-leaves/
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