Why Is the Basil Looking So Poorly?

Is it sunburn or downy mildew?
If there's sporulation on the lower leaf surface, it's downy mildew.

2013  More than a few members have lamented "What's wrong with the basil?"  In January, Cornell warned gardeners about a new destructive disease of basil, Downy Mildew which first made its appearance in the US in 2007 down in Florida.  Take a look at their article Expect and Prepare for Downy Mildew in Basil where you will find some good photos of this disease.  Compare them to what you see on your basil.  I think we have Downy Mildew in the Garden, probably made worse by the heavy rains, evening overhead watering, and the winds we've had lately.

2015  This disease, Peronospora belbahrii, first appeared in our Garden in 2013. Visit Cornell's  Vegetable MD Online: Downy Mildew in Basil for photos and strategies for dealing with this new disease.  Two ways basil is infected are through contaminated seeds and airborne spores.  Nurseries may use steam treatment of seeds to kill the DM.  The strategy for contamination via air is good spacing and keeping the leaves dry.  The spore needs 7 hours of warm moist darkness to begin growing, so lighting next to the plant may give some protection.  Once a plant is infected the rate of infection for all plants surrounding it goes up.  Therefore, as soon as the disease is spotted, pull and bag the whole plant.  The unaffected leaves can be harvested away from the Garden to avoid spreading any spores.  Freeze the leaves or make pesto. Joan's personal experience in her own garden last year was infection of all her basil except Aussie Sweetie, a basil that is best propagated vegetatively, has small leaves and grows in a columnar fashion about 2 ft high.

2016 Report occurrence of downy mildew as soon as possible at the monitoring page or via e-mail to mtm3@cornell.edu.