The following is a request by Nancy Gregory, Extension Plant Diagnostician, UD.
Please Report Suspected Downy Mildew on Basil
Downy mildew of basil is a serious new disease in the U.S, caused by the fungus-like Oomycete, Peronospora. Originally found in Uganda and then Europe, basil downy mildew was detected in Florida in 2007 and the northeast U.S. in 2008. The pathogen was reported in both field and greenhouse grown basil. The most noticeable symptom on affected plants was yellowing in between the veins, and it was thought to be a nutritional problem. Upon close examination, grayish sporulation of the downy mildew pathogen occurs on the undersides of the leaves. Basil downy mildew spores are produced in abundance and can be spread on by air currents, as well as on infected leaf material and seeds.
This disease has not been confirmed in Delaware, but has been reported in PA, NJ, NY,NC, and other Eastern states. For this reason, the University of Delaware is joining other universities in a monitoring program. If you see what you think is basil downy mildew, please send a sample to the UD Plant Diagnostic Clinic. Please refer to the web site http://ag.udel.edu/extension/pdc/ for sample guidelines, submission forms, and contact information.
Basil downy mildew. Photo by Meg McGrath, Cornell University.