Top of basil leaf exhibiting symptoms of downy mildew.
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Received from:
Dr. Pamela Roberts
Plant Pathologist
University of Florida
Southwest Florida Research and Education Center
Immokalee, FL 34142
Dr. Richard Raid
Plant Pathologist
University of Florida
Everglades Research and Education Center
Belle Glade, FL 33430
Dr. Phil Harmon
Plant Pathologist
University of Florida
Gainesville, FL 32611
Downy mildew on basil (Ocimum basilicum) was detected from three production sites in south Florida in the previous weeks. There are no reports of downy mildew on basil in Florida, however, this disease is reported on basil grown in France, Switzerland, South Africa, and Italy. Downy mildew (Peronospora sp.) that infects plants related to basil, such as the ornamental plants Coleus and Salvia (1, 2), is present in Florida, however, it is not known whether this is the same downy mildew pathogen and which particular hosts are infected by this strain although the isolate from Coleus is reported to infect basil in greenhouses in New York (3).
Symptoms initially appear as yellowing of leaves and are typically concentrated around the middle vein. The discolored area may cover most of the leaf surface. On the underside of leaves, a gray, fuzzy growth may be apparent by visual inspection. Sporangia are easily detected under magnification.
Although no fungicides are specifically labeled for this disease, some fungicides which are labeled on basil may be useful in disease management. Reducing the period of leaf wetness by avoiding overhead watering may also be helpful. The pathogen is believed to seed transmitted.