Basil downy mildew has been confirmed on the variety ‘Genovese’ in the NC State University research plots in Haywood County, North Carolina.
Basil downy mildew characteristics
- Pathogen: Basil downy mildew is caused by the fungus-like oomycete pathogen Peronospora belbahrii.
- Host crop: Basil (Ocimum basilicum)
- Host parts affected: Leaves are severely affected, which can result in yield reduction or plant death.
The disease typically begins in June and lasts throughout the growing season. Symptoms include yellowing or browning of leaves, which may be mistaken for nutritional problems. Infected leaves have many dark spores on the underside of infected leaves that can be seen without a microscope or hand lens.
- High-humidity and moisture—6 to 12 hours of moisture from morning dew, rain, or overhead irrigation.
- Cool temperatures (60°F).
The disease has not been found on the resistant sweet basil varieties ‘Thunderstruck,’ ‘Devotion,’ ‘Passion,’ and ‘Obsession.’ Planting these resistant varieties or pathogen-free seed of other varieties is essential to prevent basil downy mildew.
Some basil types are less susceptible than sweet basil varieties. Researchers have found red basil (‘Red Leaf’ and ‘Red Rubin’), Thai basil (‘Queenette’), lemon basil (‘Lemon,’ ‘Lemon Mrs. Burns,’ ‘Sweet Dani Lemon Basil’), lime basil (‘Lime’), and spice basil (‘Spice,’ ‘Blue Spice,’ ‘Blue Spice Fil,’ ‘Cinnamon’) to have less severe downy mildew symptoms.
In addition to infected seed. disease also spreads from airborne spores of Peronospora belbahrii. Help control by watering early in the mornings, so foliage dries quickly, and allowing enough space between plants to increase air circulation.
Disease control for home gardeners
Be diligent in checking basil leaves for downy mildew. Fungicides are most effective when applied in a preventive, weekly spray program. Products containing the active ingredients copper or chlorothalonil (‘Daconil’ is the trade name of one product with chlorothalonil) are the only effective products available to home gardeners. Apply to achieve good coverage on both sides of the foliage and apply the products before disease begins.
For more information:
Pest Alert: https://plantpathology.ces.ncsu.edu/2019/07/basil-downy-mildew-found-in-western-north-carolina-july-22-2019/
Basil Downy Mildew: https://content.ces.ncsu.edu/basil-downy-mildew