From Meg McGrath, Long Island Horticultural Research & Extension Center, mtm3@cornell.edu:
Downy mildew on basil plants for retail sale at garden centers has been reported this month in NJ, NY (Long Island), MA, ME, and WI as well as in SC. This disease was also reported on basil in gardens in DE and NJ. View information about these reports.
Considering this pathogen produces copious amounts of wind-dispersed spores, there is the potential for affected plants in a home garden to serve as a source of inoculum for commercial plantings of basil. Garden tomatoes infected when purchased were the source of late blight for the outbreak in 2009 in the northeast and in 2011 on Long Island.
To date an association has not been made between downy mildew occurring on garden basil plants and a commercial basil planting.
I have encountered downy mildew on my basil plants this week. I grow ` 600 plants for a farm stand in pots outside. I grow several different varieties and have found that Devotion and Prospera have fared better than other varieties. I am currently spraying with Oxidate II and Ranman to see if can be controlled. Environmental conditions in SE CT have been very conducive to fungal growth (~ 10-12 days of high temperature and very high humidity) during the past several weeks. Until 3-4 days ago the plants were very healthy and robust. The rapidity of onset of this disease still amazes me. I lost my entire crop 3-4 years ago. Samples were analyzed and confirmed at the Ag. station in New Haven CT at that time.