Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Herb growers should be on the lookout for basil downy mildew. It is now showing up in the region, including Delaware. There was some found earlier in the summer on transplants but now it is spreading by airborne spores called sporangia. Often the symptoms appear in the upper canopy as light yellow blotches and it is difficult to see in the early stages. These light blotches continue to turn yellow, brown then black. The fungus can be seen on the lower leaf surface as fine tufts of fungus growth as well as the sporangia that blow in the wind and spread the disease. The only fungicides that are labeled are the phosphorus acid or phosphonate fungicides that have herbs on the label which include Prophyt and K-Phite. Increase air circulation and remove badly infected plants. There is more info on the Plant Clinic website http://ag.udel.edu/extension/pdc/index.htm. The following photos are courtesy of Meg McGrath, Cornell University, Riverhead Long Island.
Yellowing on upper leaf surfaces
Pathogen growth and spores on lower leaf surfaces.
Close-up of pathogen growth.